Thursday was once the most profitable night of the week for NBC. But the network's prime-time ratings and fortunes have eroded dramatically in recent years, forcing network executives to rethink their strategy.

Last fall, NBC's schedule contained a curious recruit for the marquee time slot of 10 p.m. Thursday: "Rock Center With Brian Williams."

The news magazine show occupied the spot once reserved for the hallmark NBC dramas "ER," "LA Law" and "Hill Street Blues." Ratings for Williams' show were weak, attracting fewer than 4 million viewers an episode.

Worse, NBC advertising executives struggled to sell the show's commercial time to Hollywood film studios that long have spent lavishly on Thursday to promote their weekend movie releases.

This month, NBC executives shifted "Rock Center" to Friday, and its ratings grew 10 percent. NBC filled the Thursday-night slot with the medical drama "Do No Harm." Executives figured that it could do no worse - but it did. "Do No Harm" scored record-low ratings and was yanked after just two episodes.

NBC's crumbling Thursday lineup - ratings are down 20 percent from last season - illustrates the challenges facing the Comcast Corp.-owned broadcast network as it labors to reverse a decade of audience declines. NBC now attracts 5.2 million viewers Thursday, a fraction of its audience a decade ago, according to Nielsen.

Things could get worse: Two of the network's primary Thursday comedies, "The Office" and Tina Fey's "30 Rock," are ending their runs, leaving NBC with even more holes on the most profitable night of television.

"Thursday night has to be a priority for NBC," Jason Maltby, a top advertising buyer with the ad agency Mindshare, said in an interview. "It is a key night for marketers looking to drive their weekend sales."

From the late 1990s to about 2003, NBC generated as much as $800 million in annual profit from its "Must See TV" lineup, which boasted such defining shows as "Friends," "Seinfeld" and "Will & Grace." "Friends" drew more than 17 million viewers an episode in its last two seasons.

Back then, the Thursday lineup generated nearly 60 percent of NBC's prime-time revenue, according to people familiar with the network's finances who did not want to be identified divulging internal information.

Now, by some estimates, NBC's entire prime-time schedule loses substantially more than $150 million a year.

"They really need something that can ignite a night," said TV historian Tim Brooks. "But in an odd way, maybe losing 'The Office' and '30 Rock' was what needed to happen to help NBC start to turn things around."

Indeed, allowing the comedies to gracefully retire was part of NBC's strategy.

After the Philadelphia cable giant Comcast took control of NBCUniversal two years ago, new programming executives were installed at the broadcast network. The team inherited a prime-time schedule full of holes and a cupboard that was bare.

NBC soon began ramping up development to mount a turnaround, which Comcast has estimated could take five years. A year ago, as NBC put together its fall schedule, programming executives made a decision to not worry about Thursday. They worried that new shows would get flattened because the aging "30 Rock" and "The Office" were not bringing new viewers to the night. Instead, they marshaled their meager resources during the first half of the week when the network had stronger shows, including "Sunday Night Football" and the hit singing competition "The Voice."

"We had to take it one piece at a time," said Jennifer Salke, president of NBC Entertainment.

The gambit paid off in the fall with two new 10 p.m. dramas. J.J. Abrams' "Revolution" attracted nearly 13 million viewers an episode, while Dick Wolf's "Chicago Fire" averaged nearly 8 million. Even the veteran Tuesday family drama "Parenthood" experienced a lift.

NBC rocketed to first place in the important advertiser category of viewers ages 18 to 49 in the fourth quarter. But when "Sunday Night Football" ended its season and "The Voice" and "Revolution" went on hiatus, NBC's schedule came crashing down. CBS took over the top spot, and NBC is expected to end the TV season in third or fourth place.

Now, NBC executives are focusing on next fall. The network has ordered 10 drama pilots, and 17 comedy pilots - more than any of its rivals.

The network intends to fill its Thursday 10 p.m. slot with a drama, reassuring Hollywood producers that it hasn't abandoned scripted fare in that time period. The comedies are designed to restock the network's signature Thursday prime-time comedy block for the fall season.

"Thursday needs a new anchor," said Jeff Bader, NBC Entertainment's president of program planning, strategy and research. "There is going to be much more of an emphasis on Thursday night for the upcoming season than there was last year."

NBC hasn't decided whether to renew two Thursday sitcoms that have small but passionate younger-skewing audiences. "Parks and Recreation," which stars Amy Poehler as a small-town government official, musters about 4.1 million viewers. "Community," which stars Joel McHale and focuses on a cadre of misfits at a community college, draws about 3.3 million.

The network's indecision over the two shows is emblematic of a larger conundrum: How does it develop shows worthy of the legacy of "Must See TV" - smart and sassy but appealing to a broader audience, as CBS has done with its Thursday comedy "The Big Bang Theory," which draws more than 18 million viewers?

"It's an ongoing conversation that we have, and there are not easy answers," Salke said. "Would you rather have a passionate core audience of die-hard fans who won't miss an episode or develop a broader show that might not feel as special?"

Salke acknowledged that the network faces real challenges.

"The shows we are developing have to have broader appeal, something that people can relate to, but it really comes down to the writing, casting and execution," she said. "Those are the magic ingredients."