Millions of television show episodes have been streamed over the Web by broadcast networks over the past month, with "Heroes" and "Jericho" emerging as favorites among people who watch TV on their computers.

Within the past month, ABC, CBS and NBC have begun making some of their entertainment programs available for free download. Fox is set to begin after the World Series. All claim a rapidly growing appetite for news and entertainment on the Web but are reluctant to release specific details.

NBC says it has streamed 5 million individual episodes of its shows, and ABC said it streamed more than 2.5 million in the first two weeks. Those numbers are still tiny compared to the number of people who watch TV: nearly 23 million people watched "Grey's Anatomy," the most popular show on TV, the other week.

For prime-time shows, the bulk of computer viewing is done to catch up on an episode missed on TV, and is often done within a day of when the shows air, said Albert Cheng, ABC's digital chief.

"I will not lay claim to reducing productivity in the workplace," Cheng said.

ABC last fall began selling episodes of some of its popular shows through iTunes, setting off a rapid transformation that networks are still in the midst of. This Sept. 22, it began making video streams of six of its most popular shows — including "Grey's Anatomy," "Desperate Housewives" and "Lost" — available on ABC.com the day after the episodes air. Individual episodes remain available for four weeks.

The network is experimenting with shows that it believes fits the profile of an online viewer, he said. The average age of ABC's online viewers has been 29; for TV viewers, it's 46.

While the networks know how much each of their shows are being watched online, none will divulge the details. CBS and NBC revealed which of their shows rank No. 1 online — "Jericho" and "Heroes," respectively. ABC wouldn't even go that far. Cheng claimed cryptically that online viewing habits were similar to TV, which indicates top-rated "Grey's Anatomy" led online, too.

Nielsen Media Research is developing a third-party measurement system for TV viewing online, but it's at least 18 months from being ready, spokeswoman Karen Gyimesi said.

"It's a new business, and you're hesitant to give out too much information," said Jeff Gaspin, head of NBC's digital unit. "Partly it's for competitive reasons, and it's so early you don't want to make general conclusions about things."

The closest way of measuring is visits to network Web sites. The number of visitors to ABC.com jumped 224 percent between August and September, to 181 million, according to Nielsen/NetRatings. The increases were 30 percent for NBC and 3 percent for CBS.

The new "NBC Rewind" feature streamed more than 2 million videos during its first week up in late September, making NBC's Web site the most visited one of all the networks, NBC said. It has since hit the 5 million mark in total downloads, the network said.

"Heroes," the serial drama about people who discover they have superhuman powers that has been an unexpected success this season, accounts for "about half" of the downloads, Gaspin said. NBC is only making its new series available online, primarily viewing the service as a promotional device.

Many online viewers don't have the patience to sit through an entire program. NBC said the average amount of time a visitor spent on "NBC Rewind" was 20 minutes, with one quarter of the visits over 30 minutes.

CBS has made the most shows available on its "innertube" Web site, including all three of the "CSI" editions, "Survivor" and the sitcoms "How I Met Your Mother" and "The New Adventures of Old Christine." Like the other networks, the shows don't appear online until after they've been on TV everywhere.

Experimenting with different approaches, CBS made episodes of some shows available for four weeks, while all new episodes of "Survivor" and "Jericho" will be up all year.

The network won't say how many shows have streamed but did say it has grown 300 percent since the first week. Its rivals suggest CBS is hurt by having a less computer-friendly audience.

None of the networks believe online viewing has cut into television viewing. In the long run, they believe it will help.

"It's very early, but we think we're keeping people involved in shows," said Larry Kramer, CBS' digital chief. "It's not unusual for even good fans to miss one quarter or one half of (a show's) episodes. The idea here is that you can catch up on things very quickly."

Fox will soon begin streaming episodes of "Prison Break" and "Bones" online through MySpace.com and network affiliate Web sites within 24 hours of airtime, a spokeswoman said.

Networks want to know more about streaming's impact, and how much money they can make from advertising, before expanding the number of shows available. Another concern is the effect online viewing might have on lucrative syndication sales.

News video viewing via the Web is also increasing exponentially. ABC's afternoon online version of "World News" was streamed 62,000 times in February, and 226,000 times in September, the network said.

NBC said it had streamed more news video from its Web sites in September — nearly 88 million segments — than for any other month ever, even though no single big news event drove activity.

The numbers also illustrate that people are far more interested in selecting individual news videos than watching an entire show — in effect creating their own newscasts, Kramer said.