Late last year, America Online Inc. invited consumers to an all-you-can-eat buffet of computer online services. Consumers took the company at its word and showed up in droves -- and now it's pleading with them to go on a low-fat diet so the folks in the back of the line don't go hungry.

As some techno gurus see it, America Online's equivalent of a food shortage casts new doubt on the flat-rate style of pricing that America Online began in December and that has become the standard in the Internet business. It has made AOL so popular that it's often overloaded. Many of its 8 million members get busy signals when they try to connect.

It is an issue that is trying minds all over the Internet, which began as a network of academics and researchers who couldn't be bothered with thinking about bills and ticking clocks. Now, as it evolves into a mass medium, with millions of American households linking up for such things as sports scores, electronic mail and photographs, flat-rate pricing can be a mismatch.

Under flat-rate pricing, subscribers pay a fixed monthly fee, typically $19.95 as with AOL, to stay online as long as they want. Ten hours, 50 hours, 100 hours a month -- the price is the same.

You don't have to be an economist to know that when people don't pay extra for something, they often use more of it, generating shortages. AOL subscribers have found that to be the case. So did many computer users on Monday, when a World Wide Web site set up to dispense video of the presidential inauguration for free was able to meet only a fraction of the demand. Millions of people tried to connect to the site, but only thousands got in.

Flat-rate pricing on the Internet "is largely going to disappear," predicts Ted Julian, Internet research manager at International Data Corp., a market research firm. "To the extent that it remains, users will come to understand that you get what you pay for."

"I just can't imagine this flat-rate pricing lasting," said John Sidgmore, chief executive of UUNet Technologies Inc., a Fairfax company that provides Internet services. "I think it's going to be usage-sensitive, and customers will pay more for what they use."

Until users pay for what they get, some analysts say, the Internet may have a hard time maturing from an erratic Soviet-style system of surpluses and shortages to a smoothly functioning economic machine where supply matches demand.

The pressure on the circuits could easily get worse before it gets better. More and more users are buying computers that are powerful enough to send video and photographs, data-intensive material that can clog lines.

Even analysts who say flat-rate pricing is here to stay, as much a fixture of daily life as flat-rate local telephone service, predict that a variety of pricing systems will coexist with it.

Analyst Peter Krasilovsky of Arlen Communications Inc., a Bethesda market research firm that follows the online world, said flat-rate pricing is "a certain loser" by itself. But it may endure if Internet companies use it as a "loss leader" to build audiences for advertisers and markets for special services for which they can charge extra, the Internet's equivalent of premium cable TV channels.

"I'm sure in the future we'll offer {higher-capacity} services at a higher price," said Tom Evslin, who oversees AT&T's Internet service.

People sending live video, which is useless if delayed, might pay a premium to ensure instant delivery. Those sending electronic mail, which is still valuable if it's delayed awhile, would pay less.

Some companies already are dumping flat pricing. NetCom On-Line Communication Services Inc., an Internet access companys that had helped popularize the approach, last month decided to stop offering the $19.95 rate to new customers. At that rate, the company concluded it would end up losing money or providing unacceptably poor service, spokesman Curt Kundred said. "It just didn't seem to be a real rational approach to the business." CompuServe Inc. in November abandoned its experiment in flat-rate pricing, scrapping the Wow service it had introduced early last year at $17.95 a month. Wow was intended for computer novices, but it proved costly for the company when many sophisticated heavy users subscribed to take advantage of the price. America Online's woes are largely the result of its own miscalculations, as the company tells it. Weeks before the flat rate kicked in, Chief Executive Steve Case told financial analysts that AOL would "really be challenged to meet that demand and try to avoid busy signals and system sluggishness." But the surge in demand was "far greater" than what AOL expected, Case said last week. To cut the congestion, Case asked users to show restraint. "Just as you would be sensitive about using a public phone booth if others were waiting in line to use it . . . try to show some restraint at night during the next few months when we're in this transitional mode," Case said in an appeal to AOL's subscribers. The amount of time the average AOL subscriber spends online daily has doubled since September, rising to 32 minutes from 14 minutes. The total time subscribers spend online daily has risen to 4.2 million hours this month from 1.5 million in September. The resulting congestion has led subscribers to file several class action lawsuits accusing AOL of failing to deliver on its promise of unlimited usage. AOL predicts it will prevail in the suits. Yesterday, company officials met with representatives of the attorneys generals of 20 states to discuss the problems. The company also suffered another technical glitch, a 3 1/2-hour failure of its electronic mail system. Though AOL has 8 million members, it can accommodate only 280,000 of them -- about 3.5 percent -- at once, said David Gang, AOL's senior vice president for product marketing. AOL has been "drastically underequipped" compared with other Internet service providers, which as a rule of thumb can accommodate 10 percent of their subscribers at once, analyst Emily Green of Forrester Research said. AOL replies that other online companies tend to cater to heavier users. AOL has curtailed its advertising and is expanding its capacity, but it has not stopped signing up new members, as AT&T did for a time last year when consumer demand threatened to overwhelm its Internet service. "We're fully committed to unlimited prices" for "the foreseeable future," said Goldberg, but she added that the company may charge extra for certain services. Encouraging consumers to linger online is key to AOL's strategy, because it hopes to profit from advertising and retail sales transacted over its network. AOL's Gang said that it will be harder for AOL's rivals to sustain flat-rate pricing because they don't have the same potential to generate advertising and retail revenue. Telephone companies contend that all-you-can-eat Internet service is made possible in part by an indirect and involuntary subsidy from phone companies. Though Internet service providers send and receive traffic over local telephone networks, they are exempt from the access fees that long-distance telephone companies pay to use those networks to begin and end their calls. In the parlance of economists, they're "free riders." Internet companies say the phone companies benefit by selling extra phone lines to computer users. But phone companies say they are forced to make major new investments to prevent the Internet traffic from compromising telephone service. Federal regulators are considering the issue. CAPTION:AMERICA IN LINE?(This graphic was not available)