One of the most popular e-mail tools on the Internet is about to become one of the more prominent billboard locations online--dropping its price to zero as its developer bets that advertisers, not users, can pay the freight.

Qualcomm Inc.'s Eudora e-mail program, which has been available for years in a free but limited "light" version and a feature-rich, $50 "Pro" version, will make the price tag optional in its next release. In return for allowing two ads to display within the program, users will be able to use the Pro version for free.

Eudora--named after the writer Eudora Welty--is the most high-profile program yet to go from being "payware" to ad-supported.

The idea behind the change in Eudora--as with other ad-supported software--is that customers spend much more time inside one program than they ever would at any single Web page. "The trend is toward ad-supported software," said Marissa Gluck, an analyst with Jupiter Communications in New York briefed by Qualcomm on its strategy. "With e-mail it's a much more loyal and active user than someone who's just passing through a Web site."

The move also is aimed at answering intense competition. Eudora, bought by Qualcomm in 1991, has been losing market share to both free Web-based e-mail services from the likes of Yahoo and Microsoft's Hotmail (which can be used from any Internet-connected computer) as well as such free software as Netscape's Messenger and Microsoft's Outlook Express--the latter included with every copy of Windows. Qualcomm estimates that 16 million people use the free release, while fewer than 6 million use the $50 version.

"Seventy-five percent of our users being on the light version--that, needless to say, was not sustainable in itself," said Jeffrey Belk, Qualcomm's general manager for Eudora products. "Now we take the same audience, give them a much better client, it's better for the user and becomes a significant revenue source for Eudora."

The ads will be limited to static, soundless images, and will rotate about once a minute. The company will also invite Eudora users to fill out a marketing questionnaire when they download the product, so that advertisers can pitch themselves to particular audiences.

The program will regularly download new ads, but it will not send back data on its users' e-mail usage, nor will it tack on any sort of promotions to people's e-mail. "[We] have been pushing to keep this out of the way of users," said Eudora author Steve Dorner, now a vice president of technology at Qualcomm.

Besides the ad-supported version, users can also pay for an ad-free version or downgrade to a new light version that only includes one ad. Preview editions should be available for download today at www.eudora.com.

Gluck, the Jupiter analyst, said Qualcomm will need to offer users a good reason to "qualify" themselves in its marketing questionnaires: "You can't just ask people to fill out a survey so they can get more advertising." Without this sort of targeting data, she said advertisers would be unlikely to pay more than the going rate for Web banner ads--$10 per thousand viewings of a single ad.

One privacy advocate, however, warned users to download with care. "It's a dangerous trend," said Jason Catlett, president of Junkbusters, a Green Brook, N.J., consulting firm. He objected to both the intrusiveness of many in-program ads and the surreptitious data-collection carried out by some free programs.