Is there room for more than two networks on Thursday? We’ll find out this season, as ABC, Fox and the WB, sniffing for the slightest vulnerability in Thursday gorillas NBC and CBS, have assembled their smartest attacks on this night in years.

As the big night for movie studios looking to advertise their latest blockbuster, Thursday is a priority for every network — and hits or misses here could swing the overall fortunes of a net.

Last season, it was the strongest night of the week collectively for the six broadcasters, who commanded a 61 share of the adults 18-49 audience and a 65 share of the 18-34 crowd, according to Nielsen.

CBS, dead on Thursdays until three years ago but now the night’s most-watched net, could be in the hunt for second this season in 18-49 if “Without a Trace” can make inroads against “ER.”

And Fox would solidify its position as a legitimate challenger to demo leader NBC if it can at last do something — anything — on what has been its weakest night.

Of course, NBC has been counted out on Thursday before, but it has managed to reign as the night’s young-adults leader for 19 years now. “Friends” will be there for only 18 episodes, and the long-dominant “ER” is slowly losing its loyal audience, but the Peacock has high hopes for new 9:30 laffer “Coupling,” which if successful could give the night a much-needed lift.

Last season, NBC topped a charging CBS by 21% in adults 18-49 (8.6/22) and was particularly dominant among the younger half of the demo, lashing the Eye by 61% in adults 18-34 (8.7/25 vs. 5.4/16). CBS bested NBC in overall viewership and shaved NBC’s advantage among adults 25-54 to a slim 8% (9.0/21 vs. 8.3/19).

Thursday is actually the strongest night for three networks: In addition to NBC and CBS, there’s UPN, which despite double-digit declines the last two seasons, remains a force, especially among males 12-34 with wrestling extravaganza “WWE Smackdown.”

Several promising, distinctive series will bow on Thursday in the 2003-04 season, with each looking to serve segments of the aud not infatuated with the NBC and CBS offerings.

Fox soap “The OC,” ABC’s midseason Stephen King drama “Kingdom Hospital” and the WB’s talk/variety show “Steve Harvey’s Big Time” should all make noise, while ABC will also get some early-season traction with second-year reality skein “Extreme Makeover.”

Here’s a look at how things stack up on Thursday, hour by hour:

8-9 p.m.

The shorter season of “Friends” creates an opportunity for other shows in the opening hour — especially during the first half of the season. CBS’ downtrending but still potent “Survivor” figures to put up better numbers with its much-anticipated “all-star” edition beginning in February.

The hour’s two new dramas, ABC’s “Threat Matrix” and Fox’s “Tru Calling,” should skew very differently, with the former older and more male. “Matrix” isn’t great, but it doesn’t have to do much to improve on last year’s “Dinotopia.”

“Tru Calling” gets to bow after the other shows — and following heavy promos during baseball — and it could be a sleeper hit among 12-34s.

The WB is going for the comedy aud not watching NBC with the solid combo of Harvey’s new show and the quietly effective “Jamie Kennedy Experiment.” Frog hasn’t been this strong here since “Gilmore Girls” three years ago.

9-10 p.m.

CBS drama “CSI,” TV’s most-watched program, likely peaked last season, but it’s not likely to drop much. Fox’s “OC,” picking up ratings steam in its summer run, and ABC’s “Extreme Makeover” could chip away at the young end of the aud (12-34) of both “CSI” and NBC’s comedies.

The wild card in the hour is “Coupling.” Auds will certainly sample the sexy sitcom, but will only stay if they like the characters. It would also be nice if it draws some men to NBC, whose Thursday is very femme.

The WB, with unproven “What I Like About You” and run-of-the-mill “Run of the House,” is in trouble. Look for a new mix of scripted and reality here at midseason.

ABC’s “Kingdom Hospital” is set for February — a daring sked move but potentially a good one, especially if Fox’s “OC” doesn’t cut it as the drama alternative in the hour.

10-11 p.m.

“ER” remains a show without a major star presence and will likely decline again this season. Last season, it topped “Without a Trace” by an average of 88% in adults 18-49, 69% in 25-54 and 33% in total viewers — margins that figure to narrow a bit this season.

It would be a strong statement for “Trace,” the Eye’s best show in the hour since “Knots Landing,” if it could grow in its second season even if lead-in “CSI” slips a bit.

ABC’s “Primetime Thursday” remains an afterthought, although a stronger lead-in will help its visibility.

Bottom line

NBC will stay on top in 18-49, but if “Coupling” doesn’t click and the Eye’s all-star “Survivor” takes off in the second half of the season, CBS could catch NBC in 25-54.

As for the other nets, there is potential for growth at ABC and Fox, and perhaps the WB.