SAN FRANCISCO — Yahoo Inc. on Wednesday will begin testing a sleeker version of its free e-mail service, shifting to a more dynamic design that mimics the look and feel of a computer desktop application like Microsoft Corp.'s Outlook.

The company plans to invite a "sizable" portion of its current e-mail accountholders to experiment with the retooled service, said Yahoo spokeswoman Karen Mahon, who declined to be more specific.

If the test goes well, all of Yahoo's e-mail users — an audience that spans tens of millions — eventually will be converted to the new system.

Yahoo imported most of the changes from Oddpost, an e-mail startup the company bought for an undisclosed amount last year.

The overhaul, described as the most extensive since Yahoo began offering free e-mail accounts eight years ago, represents the latest salvo in a technological tug-of-war for online traffic.

For the past two years, Yahoo and its main Internet rivals — Google Inc., AOL and Microsoft's MSN.com — have been unveiling a series of upgrades aimed at attracting and retaining their Web audiences so they remain appealing outlets for advertisers.

Google, which runs the Internet's most popular search engine, shook things up in the e-mail market last year by introducing a free service that included 250 times more storage than some of its rivals. Yahoo and MSN subsequently matched Google, which responded by more than doubling its e-mail storage limit to 2.5 gigabytes.

More recently, the major e-mail providers have been introducing other bells and whistles to keep their users happy and coming back for more ads. Yahoo's upgrade follows recent AOL improvements meant to make its e-mail service quicker and easier to use.

"Last year was the year of storage in e-mail, but now the real competition seems to be about who has the coolest user interfaces," Radicati Group analyst Marcel Nienhuis said.

Yahoo's e-mail service is currently leading the pack, with 63.6 million unique U.S. visitors during July, according to the most recent figures from comScore Media Metrix, a research firm. AOL ranked second with 48.7 million visitors followed by MSN's Hotmail (44.4 million), Comcast Corp.'s Webmail (5.6 million) and Google's Gmail (5.4 million).

With its changes, Yahoo's e-mail will look more like a traditional inbox that operates through a software program installed on a computer hard drive instead of being hosted on the Internet. Yet Yahoo's redesigned service still relies on a Web browser and won't require its users to install anything on their computers.

Using "dynamic" html, Yahoo's e-mail accounts will feature an inbox containing all e-mails on the top of the page with a separate pane for reading e-mail below it. The feature is meant to enable users to scroll through an e-mail folder without having to click back and forth between Web pages.

Yahoo's test audience also will use a computer mouse to "drag and drop" e-mails from one folder to another and search all the content, including attachments, stored in the inbox.

"Our competition has been doing some interesting things in e-mail, but we think we have leapfrogged them all with all these new features," said Ethan Diamond, an Oddpost co-founder who works for Yahoo as a director of product management.