Three NBC series fell to all-time ratings lows in the key 18-49 demo Thursday night, based on Nielsen overnights. TVbytheNumbers.com reports that “Community,” “Parks and Recreation” — the first of two back-to-back episodes — and “1600 Penn” all dipped to historic lows.

At the other end of the ratings spectrum, Fox saw some improvement from its sputtering “American Idol” to win the night. The two-hour “Idol” rose 8% from an hour episode a week ago to register a 3.9 average in viewers 18-49.

The highest-rated show of the night, by a sizable margin, was second-place CBS’s “The Big Bang Theory” with a 5.4 average in the 18-49 demo, a 2% dip from last week. CBS’s “Two and a Half Men” came in with a 3.9, up 3%, while “Person of Interest” slipped 3% to a 2.9 and “Elementary” delivered a 2.3, even with a week ago.

ABC’s results were mixed, but the most troubling was a 21% plunge for the new drama “Zero Hour” from last week’s series premiere, leaving the show with a modest 1.1 average in 18-49. With “Grey’s Anatomy” up 7% to a 3.0 and “Scandal” holding steady with a 2.7, ABC had to settle for fourth place for prime time overall behind Univision.

NBC came in a distant fifth overall, in a tighter race with sixth-place CW than with the other broadcast nets. “Community” was down 8% from last week with a 1.1 average in viewers 18-49; “Parks and Recreation’s” first episode dropped 13% to a 1.3; and “1600 Penn” was off 9% to a 1.0 to cap off the night of historic lows.

The second episode of “Parks and Recreation” managed a 1.4 in the 18-49 demo, but that was well below the 2.0 delivered last week by the slot’s usual occupant, “The Office.”

For prime time overall, Fox led the way in adults 18-49 with an average rating of 3.9, followed by CBS (3.3), Univision (2.5), ABC (2.2), NBC (1.2) and CW (0.9). The battle for total viewers was tight at the top, with CBS edging Fox — 13.52 million to 13.50 million — followed by ABC (7.2 million), Univision (5.8 million), NBC (3.2 million) and CW (2.2 million).