The highly touted Web-based drama series quarterlife proved a network television flop in its NBC debut, drawing the network's worst ratings for its time slot in at least 20 years, Nielsen Media Research reported on Wednesday.

NBC had high hopes for the made-for-Internet series, a show about young adults designed to appeal to the very audience group -- viewers aged 18 to 49 -- prized most by television advertisers.

But the show's dismal performance in its prime-time network launch on Tuesday threw its immediate future into doubt at the General Electric Co.-owned network, where a source said the series could end up canceled before its next airing.

An NBC spokeswoman said quarterlife officially remained on the network schedule for now. It had been slated to move to Sundays on March 2.

But its initial broadcast on Tuesday ranked a distant third place for the 10 p.m. hour, averaging 3.1 million viewers and a meager 1.3 rating among the 18-49 crowd, the lowest for NBC in that time period since Nielsen began measuring TV viewing by age with people meters in 1987.

By comparison, NBC's usual Tuesday 10 p.m. show, Law & Order: Special Victims Unit, has consistently led the hour with a 4.5 rating among adults 18-49 and more than 12 million viewers overall.

quarterlife, dramatizing the urban lives of six young artists, was originally created for the social-networking site by Marshall Herskovitz and Ed Zwick, Emmy-winning producers of thirtysomething and My So-Called Life.

Consisting of 36 eight-minute webisodes, the series began running on MySpace.TVcom and in November, with two new segments appearing online each week.

NBC made headlines when it announced in the midst of the Hollywood writers strike it was picking up the series as a mid-season replacement show, and has heavily promoted the drama in the run-up to its prime-time launch.

At the time, quarterlife was touted as a new model for the development of video entertainment, marking the first program to originate independently online before moving to a major broadcast outlet.

NBC Entertainment co-chairman Ben Silverman acknowledged in remarks to The Hollywood Reporter on Wednesday the experiment did not live up to expectations, but was so worth the try.

The Web site traffic went up a huge amount, and we continue to try new things and new models, he said. It's very inexpensive but we hoped for higher ratings.

The show's network debut may have suffered somewhat from viewer fatigue among its target audience given that the series has already run online and cable network MTV aired segments of it the day of its NBC premiere.

As expected, American Idol on Fox ranked as Tuesday's biggest broadcast, averaging 29 million viewers overall.