As Web marketing starts to show significant results, mainstream advertisers are jumping in.

It began as an experiment. Bristol-Myers Squibb Co. wanted to see if the Internet was all it was cracked up to be when it came to plugging products and services. So during the 1997 tax season, the giant drug company teamed up with financial software maker Intuit Inc. and launched an online advertising campaign extolling Excedrin as ''the tax headache medicine.'' For 30 days, Bristol-Myers ran ads on financial Web sites offering a free sample of Excedrin to Net surfers who clicked on the ad and typed in their name and address.

The response was as good as any elixir. In just one month, Bristol-Myers added 30,000 new names to its customer list--some 1,000 per day and triple the company's best-case scenario. What's more, the cost of obtaining those names was only half that of traditional marketing methods. ''I don't think anyone can deny the long-term potential of the Web as a marketing tool,'' concludes Margaret Kelly, Bristol-Myers' vice-president for advertising services. Adds Seth Goldstein, president of CKS Site Specific in New York, which produced the campaign: ''We turned a packaged-goods company into QVC.''

Ah, the power of advertising--on the Net, that is, and at long last.In the past nine months, swarms of new ad banners, buttons, sponsorships, even TV-like commercials in which a car explodes onto your computer screen and speeds away, are now splashing their way across the World Wide Web. In the first quarter of 1997, Internet ad spending hit $133 million--still just a fraction of the billions spent on TV advertising, but a remarkable fivefold increase over the same period last year. What's more, analysts now say that Web advertising will approach the $1 billion milestone by yearend  ''Net advertising,'' says Andrew S. Grove, CEO of computer chip powerhouse Intel Corp., ''is becoming a big deal.''

Not surprisingly, tech giants, already the biggest online ad spenders, are planning to pump up the volume even louder. In August, Intel announced that it will extend its cooperative-advertising program, which subsidizes ad dollars spent by its industry partners, to include Web promotions--a move that analysts say will boost all online ad spending some 40% in 1998. IBM says it's placing ads on 500 Web sites this year, a tactic that will balloon its Web advertising budget 300% above its 1996 level. And software giant Microsoft Corp. is upping its Net outlay by as much as 70% in fiscal year 1998, on top of the $24 million it spent in fiscal 997, which ended in June. ''This year, Web advertising is a permanent part of our marketing constellation,'' says Microsoft's John Zagula, director of marketing for desktop applications.

But the biggest change yet in this surge of cyber ads is that it's not all coming from computerdom. Today, makers of everything from Toyota Corollas to Kellogg's Corn Pops are hawking their wares over the Web. Yahoo! Inc., the first Net startup to turn a profit just from ad sales, says its mix of advertisers has gone from 85% tech in 1995 to almost 80% consumer brands today. Excite Inc., the No.2 search company, has seen its proportion of nontech advertisers grow from 38% to 59% in just the past six months.

Proof is in the list of top-20 advertisers in the first three months of this year.According to New York-based researcher Jupiter Communications Inc., online high rollers now include General Motors, American Express, Walt Disney, Procter & Gamble, and publishers such as Dow Jones. ''The Internet is no longer in the realm of experiment,'' says Farris Khan, Internet advertising coordinator for Saturn Corp. ''It's part of our mainstream advertising, like television or print.''

In ad parlance, the Net has come a long way, baby. Just a year ago, Web site operators were wringing their hands over disappointing ad sales--$301 million for the full year, hardly a flyspeck amid the $175 billion pumped into all U.S. advertising in 1996. This was even more distressing since ad sales had been expected to be the Internet's cash machine--a rich and ready supply of revenues that could support all the jazzy Web 'zines, flashy entertainment guides, elaborate sports sites, even Internet E-mail.

So what changed? Why is advertising on the Net starting to click? For starters, the sheer number of Netizens prowling the Web, some 24 million today, is becoming too large for companies to ignore. Forrester Research Inc. expects that number to double to 52 million by 2000, putting the Web on a fast track to coveted mass-media status. What's more, in the past two years, the Net has gone from being a haven for nerds and academics to a hangout for professionals, teenagers, and grandmothers alike. This rich demographic shift, coupled with technology that promises to make Net ads almost as much of a ''must see'' as those on TV, have finally turned the Web into a hip place to pitch.

TIGHT FOCUS. At the same time, the buzz about how the Net's technology makes it possible to target specific customers is becoming a reality. Unlike a TV ad on say, Seinfeld, which is aimed broadly at the cool, thirtysomething crowd, a cyber-promo can zero in on Netizens who live in a specific part of town, are female, and who have shown an interest in certain topics or products.

Toothpaste maker Mentadent got a taste of that when it recently launched a two-week test on the Net with PointCast Inc., an information network based in Sunnyvale, Calif. But first, PointCast sent E-mails to random visitors to its site asking about their dental hygiene habits: 72% said they brushed twice a day, and 33% brushed at the office. Mentadent was then convinced the PointCast audience was ripe for its pitch. The result: Double the average number of people clicked on the ad for more info. ''The Internet is a marketer's dream,'' says analyst Peter Storck of Jupiter Communications.

And a nightmare for some Netizens. While advertisers drool over the ability to target specific customers, Web surfers who preciously guard their privacy may find it unnerving and start to balk, throwing a monkey wrench into this newfound ad machine. A new BUSINESS WEEK/Harris Poll, for example, found that 65% of those surveyed say they are not willing to share personal information about themselves so online ads can be targeted to their tastes. ''If I think someone is tracking me, it bothers me,'' says Matthew Hart, a 23-year-old Los Angeles Web surfer who is willing to give out information when he chooses.

SECRET STASH. Many Netizens may not even know their Web habits are being monitored. Much of this is done through a technology called ''cookies,''  which is like an electronic footprint that chronicles your movements on a particular Web site--what ads you saw or what you clicked on for more info. That data is stored in a ''cookie file'' in your browser. The next time you drop by the same Web site, the server picks up your footprint and gathers more info that can then be shared with advertisers.

Privacy advocates fret about cookies, but Web site operators insist they're not a problem since surfers can disable them by clicking on a browser option to reject cookies. Indeed, as cybernauts become more familiar with the Net, they will either get accustomed to handing out personal data or slam the door. ''If you're under the impression you're anonymous when surfing the Internet, you're wrong,''  says Dennis L. Wilson, a 31-year-old intellectual-property lawyer in Los Angeles who cruises the Net regularly. ''I don't have a problem with that. It's how you get advertisers to pay.''

That's the attitude admeisters are banking on. Netizens are opening their arms to ads in cyberspace, though sometimes grudgingly. The BUSINESS WEEK/Harris poll showed that 67% of those surveyed were not willing to pay users' fees to avoid online commercials. What's more, 57% said they ''somewhat agree'' that Net ads are useful sources for product data and information. On the flip side, 46% also said they ''somewhat agree'' that it's easy to ignore Web ads.

And that's the rub. While there is more ad activity on the Web than ever before, much of it is still crude and far from the standard-setting slickness of TV commercials. But that doesn't mean Net advertising isn't headed in that direction. To be sure, Web advertising is borrowing pages from its broadcast counterpart with the gradual addition of sound and animation and the growing use of sponsorship ads, such as Visa's backing of Restaurant Row on the GeoCities Web site, where Netizens swap recipes and chat with guest chefs.

The next step: Netmercials. The biggest attempt yet to mirror TV can be found ina new genre called '' interstitials.'' These are full-screen, in-your-face ads that pop up either in the lag time between requesting a Web page and its appearance on the screen or, much like TV commercials, between segments of shows produced exclusively for the Web.