For the last five years, NBC has been guaranteed a solid night of loyal television viewership each Thursday night. Traditionally, their Thursday-night lineup, wisely entitled Must-SeeTV, has been just that — a solid three hours of programming that sustained a following while attracting new devotees.

Originally, "Friends," "Seinfeld" and "ER," along with two other sitcoms of the season thrown in for good measure, were unbeatable by other networks. Despite their best efforts, CBS, ABC and Fox were never able to topple the powerhouse.

No matter how many rip-offs they produced, from an endless barrage of "Friends"-themed comedies to nitty-gritty, real-life dramas, the rival networks' attempts always failed. Somehow, their stars just did not have the right haircuts or their shows the right theme songs to make it.

Now, with the departure of "Seinfeld" two years ago and the ever-increasing monotony of the remaining series, the competition may finally get its chance to steal the rug out from under NBC.

Last Thursday night signaled the start of another season of Thursday night television. "Friends" returned to its usual 8 p.m. timeslot. "Frasier," being the closest thing to a "Seinfeld"-type program that NBC could find after the latter signed off the air for good, also reclaimed the 9 p.m. position in the middle of the lineup.

"ER" was absent for the night of premieres so that the network could introduce its new hopeful, "The Third Watch," and grab the attention of those viewers that neglected to check TV Guide before tuning in for the night. The plan, which usually is a successful one, entails sneaking a new show into another popular program's time-slot, so that hopefully the established audience will be too lazy to change the channel or do something else productive. Therefore, the network has established a following in one effortless swoop.

In the two other less-hyped slots, "Jesse" and the new sitcom, "Stark Raving Mad", made their season premieres. The 8:30 p.m. and 9:30 p.m. locations, despite their lead-ins, tend to get the lowest ratings. This is why it is not surprising to see NBC play musical chairs with its programming.

Past shows that have called these timeslots home include "Suddenly Susan" and "Veronica's Closet." Both have since been relocated to other nights, and both have lost a majority of their viewers.

Though the programs have survived nonetheless, other sitcoms have fared worse. Forgotten programming includes "The Single Guy" and "The Naked Truth," which both disintegrated after losing the coveted Thursday night locations. Once placed on a random night of the week, their ratings slipped so dramatically that the networks removed them from the schedule all together.

Every summer, while most people vacation and forget their troubles, television writers rack their brains to come up with new storylines to keep the shows interesting and unpredictable. What they came up with this season does not exactly fit those categories, particularly the unpredictability aspect.

On "Friends," the entire gang continues its adventure in Las Vegas, where they went to meet up with Joey in the May's season finale. Of course, Monica and Chandler, having actually sustained a relationship for an entire year, consider eloping in one of the chapels, but discover that Ross and Rachel, in a drunken haze, have beaten them to it. When they sober up and realize what they have done, they have mixed reactions.

Ross does not want to get a divorce, even if their actions were hasty, because he already has two failed marriages behind him. Meanwhile, Monica and Chandler stay together in the end, though without the ceremony to make it official. With all these couplings, it is inevitable that the two remaining friends, Phoebe and Joey, will eventually pair off. Though this was the obvious intention of the show from the very beginning, several more breakups and reconciliations must take place in order for the network to milk as much as it can from the show.

NBC is already paying each of the six co-stars a pretty penny, so it is obvious that it has a lot invested in the success of the show. Only time will tell if their investment will pay off.

On "Jesse," the title character, who refused a proposal from last season's love Diego, moves on with her life. She decides to go back to nursing school, accept a job as a nurse's aide in an infirmary and take care of her young son. However, her duties begin to become too much for her as she questions her ability to be everywhere at once.

By the end of the premiere, she has become empowered again, realizing that she can accomplish anything if she puts her mind to it. She also reunites with Diego after he returns from refusing a job relocation.

Christina Applegate, the show's star, has never been able to capture the same allure as her "Friends" counterparts. The introduction of new supporting character Kurt, an extremely cynical registered nurse at the infirmary, may provide the much-needed comic relief to sustain the show for a little while.

During an unusual episode of "Frasier," the good doctor becomes involved with a woman that bears a striking resemblance to his late mother. Of course, he and his brother begin to analyze this choice in psychological terms. Not surprisingly, the relationship dissolves by the end of the half hour, leaving him a single man ... again.

In the only comedy series premiere of the night, "Stark Raving Mad," a book editor is assigned to work under the famed-but-blocked horror novelist Ian Stark. The writer is extremely eccentric and standoffish, while his editor is quite phobic and compulsive. Though the two clash in the beginning, it is obvious that their partnership will be beneficial to them both. Their opposite natures are cause for comedy, though it is difficult to tell this from the pilot.

Though "ER" did not air, its absence is not the sole reason why the Must-See TV concept is slipping. All the shows are becoming a little too contrived at this point, a common side-effect of programming that is written exclusively for viewership.

Between the high price NBC is paying to keep the stars of its three big programs, "Friends," "Frasier" and "ER" happy, and the departure of big stars from "ER," namely George Clooney, the network is at its most vulnerable. Most likely, rival networks also will sense, and capitalize on NBC's weakness. The Thursdays may be numbered for this former powerhouse.

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