CBS' move, which is believed to be the first time a major network is offering prime-time downloads exclusively online through its own network-branded cite - CBS.com - follows a series of trials balloons in which broadcast, basic and pay cable networks have previewed episodes of series on online portals, search engines, or TV destination sites. It also follows ABC's surprising deal with Apple Computer to distribute episodes of some hit prime-time shows as paid downloads via iTunes for Apple's video iPod, though many users are simply downloading it to their PCs.

CBS began offering "Threshold" online Wednesday. It was not supported with pre-roll advertising, but ads do appear on the CBS site on which the episodes may be viewed. Each episode will be available online for three days.

This short-term, on-demand offering represents the third prong of CBS's aggressive push online--the first two being news and sports--said CBS Digital President Larry Kramer.

He added that delivering "Threshold" and CBS's other entertainment programming online presents more challenges than news or even sports. "News was easy because we make and own all of that content, and sports is more complicated because we have to deal with leagues and franchises," said Kramer, adding: "But, with entertainment the business model is far more experimental, because there are many parties who expect compensation."

Kramer said his top priorities include continuing to establish CBS.com as an online destination, moving more original content online, and aggressively building an audience through communities with mutually shared interests in CBS's programming. "That's our future," said Kramer. "We have to build content and build communities around the content we offer."

With the relaunch of CBS News in July as a "24-hour broadband network," through the relaunch of CBS Sports as CBS SportsLine.com in September, to "Threshold," Kramer said CBS is aiming for a far younger, more tech-savvy demographic.

Asked whether CBS's migration online unlocks new opportunities for its advertisers, Kramer responded: "Absolutely." He said an average of 15 years divides CBS News' television viewers and its online viewers.

Kramer also said he has had talks with Apple about distributing CBS shows on iTunes. When Apple CEO Steve Jobs unveiled the video iPod in early October, Apple had already struck a distribution deal with rival network ABC. Consumers can now buy episodes of five of ABC's premiere shows, including "Lost" and "Desperate Housewives," through iTunes for $1.99 each.

Subscription and ad-supported models, as well as selling content piecemeal via iTunes, are all options being considered by CBS Digital. Kramer said a partnership with Apple is a strong possibility as long as the "economic proposal is fair enough."

Perhaps a indication of things to come, soon after the video iPod debuted, Kramer announced that CBS would distribute its free podcasts, from "60 Minutes" to the long-running soap "Guiding Light," on Apple's iTunes Music Store.

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