With hits like "Friends," NBC once ruled Thursday nights.

NEW YORK (CNN/Money) - NBC once seemed untouchable on Thursday nights, the biggest evening of the week for young adult television viewers and a huge draw for key advertisers, notably movie studios looking to spur box office sales.

To see how times have changed for NBC -- and Thursday nights in particular -- consider the fall television schedules that the Big Four broadcast networks have announced this week as they gear up to sell some $9 billion worth of advertising.

"Thursday is going to be a very, very competitive night," predicted Brad Adgate, the senior vice-president of corporate research at New York marketing firm Horizon Media.

Adgate is referring to plans by ABC and The WB, both with weak Thursday night ratings, to shift two of their more popular shows to the night.

In announcing a fall lineup that included a dozen new series, ABC said Wednesday that it will move its spy drama "Alias" to Thursdays at 8 p.m. The news came just hours after The WB told prospective advertisers that its hit series "Smallville" will now air in the 8 p.m. time slot.

ABC is a Walt Disney property. Time Warner owns The WB and CNN/Money.

Both shows will go up against "Joey" on NBC and "Survivor" on CBS.

"The WB and ABC are taking some of their more familiar shows and moving them to Thursday just points out its going to be an exceedingly competitive time period," said Adgate, "and points out the popularity of Thursday night as a source of advertising revenues."

Not long ago, the major networks all but ceded Thursday nights to NBC.

NBC ceded its Thursday night reign two years ago to CBS. Before that, the General Electric network dominated the night for most of the previous two decades with a solid lineup of comedies such as "The Cosby Show," "Family Ties," "Cheers," "Frasier," and then "Friends."

Then, CBS mounted a serious challenge to NBC a few years ago by airing ratings winners "Survivor" and "CSI" opposite NBC's hit Thursday night lineup. CBS took the ratings lead on Thursday nights two years ago, finished in first place last year, and is on track to repeat when the current season soon ends.

This fall the Viacom network is not messing with its tried-and-true formula. The CBS lineup with start with "Survivor: Guatemala" at 8 p.m., followed by "CSI: Crime Scene Investigation" an hour later.

NBC is sticking with its current Thursday night strategy too, with "Joey" and "Will & Grace" airing first. "The Apprentice" will come at 9 p.m., followed by "ER."

ABC, meanwhile, is looking to use "Alias," featuring Jennifer Garner as a double agent, to launch "The Night Stalker." A remake of a 1970s cult series about a journalist who investigates crimes, "The Night Stalker" is one of several new dramas with supernatural themes as networks look to capitalize on the popularity of current sci-fi shows "Lost" on ABC and "Medium" on NBC.

On Wednesday, Stephen McPherson, the head of ABC's primetime programming, told advertisers that he is focused on ABC's weaker nights now after spending the last year building its primetime ratings on Sundays and Wednesdays.

ABC's Thursday night lineup is a "nice step forward for us," said McPherson. "We want to be in business on Thursday night."

Shari Anne Brill, the programming director at ad-buying firm Carat USA, agrees that Thursday night has become a ratings battleground. But she doesn't think the night is up for grabs.

"Think of CBS as Democrats and NBC as Republicans," said Brill. ABC, The WB and Fox are like third-party candidates. Each will draw young adult viewers away from CBS and NBC, but at the end of the day it will still be a two-way race between CBS and NBC.

"The other networks," she said, "will chip away at the 18-to-49 year-old piece."

Even McPherson conceded that ABC doesn't have a shot at dominating Thursday nights.

"Are we looking to win Thursday night?" said McPherson. "Absolutely not." Still, he considers "Alias" and "The Night Stalker" on Thursday nights "a great two-hour block."

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