"The Civil War" was a big winner in last week's ratings race, while "Twin Peaks" was a big loser.

Although national figures are not available for PBS programming (not all PBS stations run the shows at the same time), "The Civil War" garnered a 9 rating in the nation's 24 largest cities - an increase of more than 300 percent from a normal viewing week. PBS estimates the 11-hour miniseries was seen by 14 million viewers each night.The show put a big dent the combined share of ABC, CBS and NBC, dropping it from 68 percent two weeks ago to 63 percent last week.

The much-ballyhooed return of "Twin Peaks" failed to light much of a fire in the Nielsens. Sunday night's two-hour movie finished in a tie for 41st for the week, and finished behind "Perry Mason," "The Face of Fear" and "Married . . . With Children" in head-to-head competition.

ABC has more problems than just "Peaks," however. Another highly touted show, "Cop Rock," managed just a weak ranking of 56th for the week.

And one trend concerns all four networks - none of their new shows are looking like big hits. The only member of the class of 1990 in the Top 20 was "America's Funniest People," which rode the coattails of "America's Funniest Home Videos" to No. 13 - but 700,000 homes tuned out after "AFHV."

After that, the best any new shows could do were "Fresh Prince" at No. 26 and "Married People" and "Uncle Buck" tied for 37th.

For the week, NBC moved back into the top spot with a 12.9 rating and a 22 share. CBS, which finished first a week ago, dropped to second with a 12.3/21. ABC was third with a 12.0/20. Each ratings point represents 931,000 homes, and a share is the percentage of homes actually watching TV that are tuned in to a particular network.

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`TWIN PEAKS' IN TROUBLE?: Despite the less-than-spectacular ratings, "Twin Peaks" may not be in any danger yet.

Networks don't just care about how many people are watching, they care about who is watching. If a show pulls in the young, affluent audience advertisers like, it can survive relatively low ratings.

For example, "St. Elsewhere" was never a Top 20 - or even Top 30 - show, but it lasted five years. And if you look below "Peaks" on this week's list, you'll see "thirtysomething" at No. 46 - and this is that show's fourth season.

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THE TOP 10: 1. "Cheers," NBC; 2. "60 Minutes," CBS; 3. "The Cosby Show," NBC; 4. "Murphy Brown," CBS; 5. (tie) "Designing Women," CBS and "The Golden Girls," NBC; 7. "Roseanne," ABC; 8. "Empty Nest," NBC; 9. "America's Funniest Home Videos," ABC; 10. "A Different World," NBC.- ***

TONIGHT ON THE TUBE: CBS spent more than $1 billion for the baseball playoffs and World Series. The network will start to see some return on that investment when the Pittsburgh Pirates travel to Cincinnati to take on the Reds in Game 1 of the National League Championship Series (6 p.m., Ch. 5).

Also, Beverly Hills, 90210 (7:30 p.m., Ch. 13) debuts on Fox. It's the story of a Midwestern family - well, mostly about 16-year-old twins Brandon and Brenda - who move to the chic community and experience culture shock.

Elsewhere, there's a funny episode of Cheers (8 p.m., Ch. 2); the 10-part series Race to Save the Planet (8 p.m., Ch. 7) - a look at how man can help cure the Earth's environmental ills - debuts; Grand (8:30 p.m., Ch. 2) returns for its second season in the aftermath of last spring's tornado; Law & Order (9 p.m., Ch. 2) deals with murder, AIDS and euthanasia; and Mystery! (9 p.m., Ch. 7) has an excellent episode - "Agatha Christie's Poirot: The Incredible Theft."

- LOOKING TOWARD FRIDAY: Game 2 of the National League Championship Series is an afternoon affair (1 p.m., Ch. 5); Sam has a chance to save his brother's life on Quantum Leap (7 p.m., Ch. 2); there's lots of great music on Great Performances (8 p.m., Ch. 7) "Spike & Co: Do It A Cappella"; Norm and Cliff visit Wings (8:30 p.m., Ch. 2); 20/20 (9 p.m., Ch. 5) follows up on a report last spring about Romanian orphans.