Take the Web site for You Don't Know Jack. This irreverent quiz show, a favorite among CD-ROM game players, now offers its game online, where companies such as Seven Up, Hugo Boss, and 20th Century Fox Film reach out to the site's 150,000 players each month. There, they show 15-second ads that run the gamut from previews of movies such as Speed 2 to an insect riding a motorcycle across your screen hawking Cuervo Gold. The cost: $14,000 for every 100,000 games played still a fraction of what companies pay for a slot on TV.  ''Advertisers see this and say: 'Now I see what the Internet can do for me,''' says Chris Deyo of Berkeley Systems Inc., which produces the game show. So far, reaction from Web surfers is mixed. While interstitials have more pizzazz than banners, some Netizens think more animation can be downright irritating. AT&T found that out the hard way. In mid-September, the telecommunications giant silenced an ad showing a young, sassy girl knocking on a door and asking to be let in. Again and again, she knocked until the Web surfer clicked on the door. She then walked through the doorway and said she was off to college, and users then got info about AT&T's communications services. But many found her voice annoying. 'In a word, it backfired,'' says an AT&T spokesman.

Today, in-your-face ads account for just 5% of the pitches made online. Instead, the ads of choice are banners, which amount to 80% of all electronic plugs.Typically, banners are plastered across the top or bottom of a Web page, much like a billboard on the Info Highway. A click on one of these ads usually whisks the Netizen off to the Web site of that company, where more info can be found.

BURNING RUBBER. No company knows the power of banner ads better than Toyota. The auto maker slaps these road signs all over the Net. For the 12 months ended in May, Toyota says 152,000 Web users typed in their name and address and requested a brochure or video about a car. Toyota later matched those names with buyers at its dealerships. It turned out the Web ads led to the sale of 7,329 cars--a remarkable 5% conversion rate. ''The Internet,'' says Jon Bucci, national interactive communications manager for Toyota Motor Sales USA Inc., ''is the No.1 lead generator for Toyota.''

But the static, wallpaper-like approach of banners also draws criticism as a cyber-snore. ''There's a certain coldness to the Web. It's quiet and flat,'' says Stephen Block, AT&T's director of brand and interactive advertising. Adds Jupiter's Storck: ''I have yet to see a banner that can make me cry the way an AT&T television ad can.''

Net advertisers are working on that. Today banner ads are getting a makeover with animation, music, even games that help advertisers build brand awareness. Cereal powerhouse Kellogg Co., for example, is running a series of interactive games about Corn Pops and Pop Tarts inside banners. One, called ''Mean Granny,'' is aimed at kids on the Sony PlayStation site where players try to keep grandma from getting to the Corn Pops.

No matter what form they take, online ads are being targeted to more precise audiences than in any other medium. Using keywords is one effective way. These are words surfers type into a search engine to generate a list of sites about a particular topic. These days, companies are paying for the rights to easy-to-remember words such as ''beer,'' which instantly triggers an ad from Miller Brewing on Yahoo!'s site. IBM pays for some 200 words, including laptop, notebook, and Nagano, site of the 1998 Winter Olympics. In an especially aggressive move, Big Blue once bought the word of its major rival, ''Microsoft.''

An even more pointed route to the ideal consumer is through an approach by Infoseek Corp., a Sunnyvale (Calif.) Internet search service. Infoseek is using artificial intelligence software developed by Aptex Software Inc., a San Diego company, to create a program called Ultramatch. This automatically takes cookie data and sorts them into 20 categories of users, such as those interested in entertainment or sports.

Customer PC Financial Network, an online brokerage, was so intrigued by the idea that it did an experiment. It ran banner ads in Infoseek's finance area and on the keyword ''finance.'' It also directed the same ads to Ultramatch's business users--tho seeking company news, stock quotes, or investment info. The result: PCFN got an 8% to 12% response rate from the Ultramatch ads, vs. less than 1.5% for banners. ''It's not how many people you reach,'' says Keith Halloran, the company's vice-president for marketing. ''It's how many of the right people you reach.''

That's the secret of E*Trade Group Inc.'s success on the Net. The Palo Alto (Calif.) online brokerage integrates its print and TV campaigns with its efforts on the Web. E*Trade is spending about 7% of its media budget on the Net, but it plans to increase that to 15% next year after the results it has had so far. In May, E*Trade began an ad campaign on Yahoo!'s finance site. In the first ten days, the ads generated some 3,118 leads for new accounts. Today, 17% of all E*Trade's leads come from online advertising. ''A media strategy without the Internet is suicidal,'' says CEO Christos M. Cotsakos.

SCRAMBLING. No wonder, then, that marketers from Silicon Valley to Madison Avenue are now scrambling to take advantage of the Net. ''I'd say 75% of our clients are asking us how they can use the Internet as an ad vehicle,'' says Greg Smith, director of strategic services at Darwin Digital, a dedicated Web-advertising unit of Saatchi & Saatchi ).

Indeed, in contrast with the ho-hum approach Madison Avenue once took toward the online world, it is now going all out to make it a core part of the business. Early this year, Ogilvy & Mather established an electronic arm, OglivyOne Interactive, that caters to clients such as IBM. Not to be outdone, giants such as Young & Rubicam and Leo Burnett are joining Saatchi in setting up their own shops to focus on the Net. Says OgilvyOne's J. Sandom: ''We need to be on the stick.''

Huge billings are the incentive. According to Jupiter, online advertising revenues will soar in 1997, to $940 million, more than triple last year, but still less than 1% of all ad spending this year. Jupiter predicts that by 2002, Net ad revenues will approach $8 billion--some 4.1% of total ad budgets.

Some online operators are already watching the ad dollars roll in. Netscape Communications Corp. nabbed $24.1 million from ads in its most recent quarter, triple the same period a year ago. And Yahoo!'s revenues of $23 million in the past two quarters have surpassed its take for all of last year. So have its advertisers, which totaled 900 in its most recent quarter--nearly nine times the number it had in early 1996. ''Whatever the growth is this year, it'll be twice or three times that next year,'' says Marc L. Andreessen, Netscape's cofounder.

Revenues are jumping in part because the most popular Web sites are starting to command more money per ad. In general, advertisers pay a fee for every 1,000 times their ads are displayed, known as ''cost per thousand,'' or CPM. The average rate for banner ads is $17, says Forrester Research, but search-engine company Lycos charges $20 to $22 to gain access to its 15 million visitors a month. That compares sharply with ad rates for TV, for example, which are $5 to $6 per thousand, while consumer magazines such as Cosmopolitan can command $35. Net ads carry higher fees than TV because they are able to target preferred customers. ''The technology has matured so much that if you just put your zip code in once, we can tag you,'' says Halsey Minor, CEO of CNET Inc., a San Francisco interactive-media company.

CNET is doing its part to hurry along the process. On Sept. 22, it launched Snap! Online, a consumer-information service that is going head-to-head with America Online Inc. Minor says Snap!'s charter advertisers, such as American Express Financial Services and Visa USA, will receive a first-of-its-kind monthly statement about the ages of the people who clicked on their ads, what part of the country they came from, and a slew of other demographic statistics they are willing to hand over. Yahoo!, Excite, AOL, and others are working up similar user profiles.

Key to the Net's arrival as a hot ad spot, however, is the establishment of independent measurement tools to gauge the effectiveness of ads. Accounting giants Ernst & Young, Coopers & Lybrand, and Price Waterhouse have recently begun auditing claims made by Web sites about their audiences and the number of impressions an advertiser gets. This month, Microsoft will begin sending its advertisers quarterly reports of all its Web sites, audited by Coopers & Lybrand. And on Sept. 9, the New York-based Internet Advertising Bureau issued guidelines to create standards for Internet advertising.

If advertisers still aren't satisfied, they can follow the path carved out by marketing heavyweight Procter & Gamble. It was one of the first companies to pay a Web site operator for ads only when someone clicked on them. ''[Advertisers] like to tie somebody else's feet to the fire, saying: 'You're my partner, you're going to deliver the ads, and you're going to have a vested interest in increasing the performance of them, just like I do,''' says Yahoo! CEO Tim Koogle.

That ability of advertisers to hold Web sites accountable is just what experts say will ultimately make the Internet as easy an ad sell as a spot on Seinfeld. Indeed, Web site operators are out to prove to advertisers that, like Visa, the Net ''is everywhere you want to be.''