Channel 20, which loses "The Arsenio Hall Show" to Channel 9 following the Friday, Dec. 29, telecast, yesterday announced that reruns of "Cheers" and "Night Court" will fill the 11 p.m.-to-midnight slot being vacated by Arsenio, starting New Year's Eve (Monday, Dec. 31) ...

Nine will air Hall at 11:30 p.m. starting Dec. 31. WUSA acquired the rights to the late-night Hall show from Paramount Domestic TV after it had been on WDCA for two years, in part because Nine also agreed to carry Paramount's companion show, "The Party Machine With Nia Peebles," following Hall at 12:30 a.m. ...

Reruns of "Night Court" and "Cheers" already air between 7 and 8 weeknights on WDCA. With some 170 episodes of the former and 200 of the latter on the shelf to draw from, WDCA will have no trouble airing different episodes at 11 ...

Channel 20 is letting the Hall hour go with a slight grumble, pointing out that Arsenio's ratings -- despite the growing success of WDCA's primetime schedule in recent months -- have been on the decline ...

In the November Nielsen book, Hall averaged a 3 rating and an 8 percent audience share, down from a 4/10 a year ago -- and this, WDCA was very quick to point out yesterday, despite the fact that its "Star Trek: The Next Generation" rerun lead-in did a 4/7 at 10 p.m., up from a 2/4 the previous November for "Star Trek" rerun lead-ins. Meow! ...

Each local ratings point represents 17,491 TV households ...

Channel 20 also announced yesterday that its monthly Sunday night at 8 public affairs show, "Making a Difference," will become a weekly in the same timeslot, starting Jan. 6 ...

The half-hour program is hosted by WDCA public affairs director Lisa Weir ...

NBC Entertainment yesterday announced timeslots for three additions to its midseason schedule it had announced last week ...

The one-hour drama "Dark Shadows," a revival of the old ABC gothic-horror series starring Ben Cross, will air at 9 p.m. Fridays starting Jan. 18. As previously announced, "Dark Shadows" will debut as a four-hour miniseries on Sunday and Monday nights, Jan. 13-14 ...

"Blossom," a sitcom starring Mayim Bialik and Ted Wass, will be sneak-previewed on Thursday, Jan. 3, at 8:30 and premiere in its regular Monday-at-8:30 timeslot on Jan. 7 ...

Bialik plays "an astute and witty teenage girl coming of age in a household with her two older brothers and divorced dad" (he's a studio musician) ...

And "Seinfeld," starring comedian Jerry Seinfeld, returns to the NBC schedule at 9:30 Wednesdays, starting Jan. 16 ...

Home Box Office and MTV Network are merging their 24-hour all-comedy services into a single cable network to be called Comedy TV ...

HBO's The Comedy Channel, which began in November 1989, and MTV's HA! The Comedy Network, which debuted April Fools' Day this year, will be merged early in 1991 ...

As direct competitors, both have had trouble finding a spot on crowded local cable systems. The Comedy Channel can be seen in 8.5 million homes, HA! in 6.5 million. The merger will give them access to 15 million, they announced yesterday ...

HBO is a subsidiary of Time Warner Inc. MTV is owned by Viacom International Inc. ...

The Jan. 1, 1991, merger will be achieved through a 50-50 joint venture with a programming lineup drawn from both HA! and The Comedy Channel. Both will continue to operate during the transition period ...

Less than a month ago two rival cable services devoted to covering the law -- In Court and American Courtroom Network -- were merged into a single American Courtroom Network ...

The president of the new advertiser-supported comedy network will be recruited from outside the two companies and the new network will be staffed by HA! and The Comedy Channel employees. HBO chairman Michael Fuchs, in a statement, said the merged comedy service "is not only a very positive move for both companies, but should have a salutary effect on the cable industry as a whole" ...

According to Erica Gruen, a cable analyst at Saatchi & Saatchi Advertising, about 12 new cable services plan start-ups in the coming months, ranging from a 24-hour talk show channel to the Cowboy Channel ...

"ABC World News Tonight With Peter Jennings" won the network race again last week with an 11.6/22, compared with a 10.0/19 for "CBS Evening News With Dan Rather" and a 9.9/19 for "NBC Nightly News With Tom Brokaw." After 49 straight weekly wins, Captain Airwaves thinks it's about time P.J. named himself "Person of the Week" ...

TV RATINGZZZZ

Following are the top 20 network prime-time shows last week, ranked according to the percentage of the nation's 92.1 milllion TV households that watched, as measured by the A.C. Nielsen Co. A share represents the percentage of actual sets-in-use tuned to a particular program when it aired.

..RATINGS.................................SHARE...NETWORK

1....22.6...Cheers...........................36.......NBC

2....20.6...60 Minutes.......................36.......CBS

3....18.2...NFL Raiders-Lions................31.......ABC

.....18.2...A Different World................29.......NBC

5....18.1...Empty Nest.......................32.......NBC

6....17.7...The Cosby Show...................29.......NBC

.....17.7...In the Heat of the Night.........28.......NBC

8....17.5...Roseanne.........................28.......ABC

9....17.2...The Golden Girls.................31.......NBC

10...16.9...Unsolved Mysteries...............28.......NBC

11...16.5...Murder, She Wrote................26.......CBS

12...16.4...Designing Women..................24.......CBS

13...15.9...L.A. Law.........................27.......NBC

14...15.8...Coach............................25.......ABC

15...15.6...Sinatra Special..................25.......CBS

.....15.6...Grand............................25.......NBC

17...15.4...America's Funniest Home Videos...24.......ABC

18...15.3...Matlock..........................25.......NBC

19...15.0...Murphy Brown.....................22.......CBS

20...14.4...Law & Order.......................26.......NBCNBC was back on top last week in the primetime race with a 12.4 rating and a 21 share, its ninth win in 13 weeks of the 1990-91 season so far ...

ABC was second by a whisker, at 11.6/20, compared with an 11.5/19 for CBS and a 6.7/11 for Fox Broadcasting ...

The Peacock network won Tuesday, Wednesday, Thursday and Saturday (no surprises there) while ABC took Monday and Friday (no surprises there) and CBS won Sunday (no surprise there, either) ...

Tiptoeing through the highlights and lowlights of the week ending Dec. 16, NBC's Bob Hope special finished 22nd (between Frank and Bob it was a very geriatric week on network TV), the network premiere of "The Witches of Eastwick" on ABC tied for 30th, "Rudolph the Red-Nosed Reindeer" on CBS tied for 36th, and NBC's "The Dreamer of Oz" was 42nd ...

ABC's New Kids on the Block Christmas special tied for 52nd, the Billboard Music Awards on Fox were 59th and the premiere of "You Take the Kids" on CBS was 66th ...

The Fox special on Marilyn Monroe tied for 73rd, the "Grammy Legends" on CBS was 78th, and "Hammer, Slammer & Slade," a failed ABC pilot, was 80th ...

What was probably a farewell glimpse of "Parenthood" on NBC Sunday night tied for 82nd ...

Among the news programs last week, aside from the always successful "60 Minutes," ABC's "20/20" was 32nd, ABC's "Primetime Live" was 58th and CBS's "48 Hours" was 61st ...

The Final and Almost Certainly Fatal Five Plus Three last week consisted of the NBC special "Saved by the Bell" in 88th, followed, in order, by Fox's "Cops," ABC's "Cop Rock" and five Fox shows -- "Totally Hidden Video," "Comic Strip Live," "Haywire," "Against the Law" and "American Chronicles," toes up in 95th and last place ...

Now This NBC News and Nippon Television Network have agreed to a joint newsgathering venture ...

It is the third such agreement between a U.S. network news division and a Japanese broadcaster this year ...

Under the five-year pact, NBC will have U.S. rights to all Japanese and international news material of NTV. The Japanese network, in turn, will have rights to the NBC-Wall Street Journal poll as well as news material from the NBC News Channel, the 24-hour affiliates news service that starts Jan. 1, "NBC Nightly News" and "Today" ...

In September, CBS News and Tokyo Broadcasting System agreed to combine their resources for worldwide news gathering starting next April. TBS gets exclusive rights to some news programs, including "CBS Evening News" ...

In October, ABC News and Worldwide Television News, a London-based TV news agency in which ABC has an 80 percent interest, signed a non-exclusive agreement to provide Fuji TV with U.S. and international news ...

After all that, TV Column fans, are you ready for some football? Monday night, the NFL tilt between the San Francisco 49ers and the L.A. Rams averaged an 18.9 national Nielsen rating and a 32 percent audience share between 9 p.m. and 12:10 a.m. on ABC ...