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ER” closed out its 15-season run on top Thursday, drawing an average of 16.4 million viewers for its two-hour finale.

The 9-11 p.m. closer averaged a healthy 6.0 rating/16 share in adults 18-49, per Nielsen.

“ER’s” signoff marked the biggest 18-49 rating for a drama series finale since Fox’s “The X-Files” wrapped its run in 2002, and the largest overall total viewer tally for a drama series finale since “Murder, She Wrote” signed off CBS in 1996. “ER’s” finale also marked a whopping 88% increase over its final-season average rating of 3.2/9 in the key demo, and NBC’s biggest numbers in the time period (sans Olympics programming) since the “Will and Grace” finale in May 2006.

NBC easily won the night with a 5.2/14 in 18-49 and 14.5 million viewers. ABC lay low in the face of the “ER” finale with repeats of “Grey’s Anatomy” and “Private Practice” from 9-11 p.m., but CBS went ahead with fresh segs of its regular lineup.

At 8 p.m., CBS’ “Survivor” (3.6/11, 11.2 m) barely beat the hourlong “ER Retrospective” spesh (3.5/10, 10.5m). Fox was No. 3 with “Bones” (2.7/8, 9.3m) while ABC trailed with the second week of new comedy “In the Motherhood” (1.6/5, 5m) and “Samantha Who” (1.6/4, 4.7m).

The 200th installment of “CSI” (3.8/10, 14.3 m) was second to the first half of “ER” at 9 p.m. Fox was respectable, given the competish, with “Hell’s Kitchen” (3.3/8, 6.9m).

CW didn’t seem affected by the “ER” hoopla, scoring its best 18-34 rating since Sept. 18. The net’s 9 p.m. drama “Supernatural” delivered its highest 18-34 demo score (1.7/5) since October 2006 on the WB.

As was oft-noted in the run-up to Thursday’s finale, the fade-out of “ER” marked the end of a primetime era, certainly for NBC. Only Fox’s “American Idol” comes close to the numbers that “ER” regularly put up in its mid-1990s heyday. (And it’s telling that “ER’s” finale scores came in far below the turnout for last week’s two installments of “American Idol.”)

Fortified by a strong ensemble that included George Clooney, Anthony Edwards, Noah Wyle, Eriq La Salle, Juliana Margulies and Sherry Stringfield, Warner Bros. TV’s “ER” was an instant sensation in its two-hour debut on a Monday night, Sept. 19, 1994. That opener drew an impressive 9.9/23 in 18-49 and 23.8 million viewers.

“ER” delivered an 11.9/31 and 23 million viewers three nights later when it settled into its regular Thursday 10 p.m. berth, filling the void in the Peacock’s powerhouse Thursday lineup left by the end of “L.A. Law” the previous season. Series never budged from its Thursday 10 p.m. timeslot.

The show hit its all-time highest numbers on the night of the “Seinfeld” finale, May 14, 1998, when the huge lead-in allowed “ER” to deliver a 25.8/57 in the demo and 47.8 million viewers, surpassing even “Seinfeld’s” aud that night of 41.3 million viewers. Nearly five years ago, the finale of NBC’s “Friends” pulled in 29.2 million viewers.

In its prime, “ER” ranked as primetime’s top-rated series in 18-49 for four of its 15 seasons (1995-96, 1996-97, 1998-99 and 1999-2000). It was TV’s top drama series for 10 seasons, until “CSI” hit its stride, and it ranked as NBC’s top drama series for 12 seasons.

Meanwhile, CBS Sports carried some strong ratings momentum into tonight’s NCAA men’s basketball tournament championship game.

Saturday’s semifinal action in primetime averaged a 9.0 household rating/18 share in Nielsen’s metered-market overnights — up 7% from last year’s 8.4/16. The first game (Michigan State’s victory over Connecticut) delivered an 8.7/18 (up 12% from last year’s 7.8/16 for Memphis-UCLA), followed by a 9.3/17 for North Carolina’s triumph over Villanova (up 2% from last year’s 9.1/16 for Kansas-North Carolina).

For the entire tournament, CBS was averaging a 6.1/13, up 7% from last year’s 5.7/12.

Tonight’s final pits Michigan State against North Carolina.

(Rick Kissell contributed to this report.)