NBC's "The Cosby Show" continues to set records, TV Column fans . . .

Thursday night's telecast produced the series' highest ratings and shares since it first premiered in September 1984 . . . and also pulled in the highest rating for any network situation comedy since CBS' "All in the Family" scored a 40.0 Nielsen rating and a 58 percent audience share the night of Jan. 15, 1972 . . .

"Cosby" registered a 39.0/56 on Thursday, Feb. 27, 1986 . . .

Jean Marsh, star of "Upstairs, Downstairs," will be at Channel 26 on Sunday night as 15 years of "Masterpiece Theatre" on PBS is celebrated that night with the aptly named special "Masterpiece Theatre: 15 Years," a highlight of the system's current national fund-raising effort . . .

C-SPAN, the public affairs cable network, which has been carrying House proceedings since 1979, will be carrying the Senate live starting June 1, when a 45-day TV experiment begins . . .

Radio coverage of the Senate could begin this week, pending installation of equipment . . .

The Senate voted 67 to 21 late Thursday to try live gavel-to-gavel coverage through July 15. The 45-day trial, however, includes a 12-day recess . . .

After a two-week review of all that has gone before, the Senate will vote July 29 on whether to make it permanent . . .

The Senate streamlined a few of its procedural rules -- notably putting new limits on filibusters -- as part of the compromise preceding approval of the radio and TV experiment . . .

Anchor Dan Rather will be back on the "CBS Evening News" tonight. Rather flew to Washington Thursday morning from Sioux Falls, S.D., after receiving word that his wife Jean had been hospitalized here. Jean Rather became ill while visiting her daughter Robin and was taken to Georgetown University Hospital Wednesday night. After a series of tests Thursday and Friday she was released, and she and her husband returned to New York Friday afternoon . . .

Local Ratingzzz

The preliminary readings on the February ratings sweeps in the Washington market are in, and when it comes to the Nielsen book at least, it's all Channel 9 in the local news races . . .

Preliminary Arbitron figures -- which we could not complete on Friday -- also show Nine ahead in most news races but by considerably smaller margins . . .

According to Nielsen, TV Column fans, Nine news programs won at noon, 5 p.m., 6 p.m. and 11 p.m. Mondays through Fridays, while "CBS Evening News With Dan Rather" on Nine registered its highest local numbers since Rather took over as anchor in March 1981 . . .

Only "CBS Morning News," which tied "Today" on Channel 4 for second, behind ABC's "Good Morning America" on Seven, was off the mark for news shows on the Gannett-owned station . . .

In the bargain, Nine's researchers are pretty sure the news programs set all-time local Nielsen viewer records in a couple of the news time slots . . .

In the Monday-Friday ratings from 7 to 9 a.m., "GMA" registered a 6.4 Nielsen rating and a 26 percent audience share, while "Today" and "Morning News" tied at 4.6/19 . . .

At noon, Nine news led with a 12.3/40, far ahead of Seven's news at 6.7/22. Four's "Here's Lucy" and Five's "Panorama" tied at 3.0/10 . . .

From 5 to 6 p.m., Nine news had a 12.8/26; the combination of "One Day at a Time" and "Alice" on Five averaged a 10.0/20; Four news had a 7.9/16; followed by Seven news with 6.2/13 and "The Dukes of Hazzard" on Twenty, with 4.6/10 . . .

From 6 to 7 p.m., Nine news rose to a 15.2/26, followed by the combination of "Gimme a Break" and "Too Close for Comfort" on Five, with a 14.2/24 average; Four news at 9.6/16; Seven local news (from 6 to 6:30) at 8.3/14; and the combination of "Diff'rent Strokes" and "What's Happening?" on Twenty, which averaged 5.4/9 . . .

At 6:30 p.m., "ABC World News Tonight With Peter Jennings" registered 9.5/16 on Seven, while at 7 p.m., Five's "Three's Company" led with a 16.2/26, followed by "CBS Evening News" on Nine, which had a 15.8/25; "NBC Nightly News With Tom Brokaw" on Four, at 8.8/14; and "The Jeffersons" on Twenty, which averaged a 4.6/7. . .

At 11 p.m., the races tightened. Nine averaged a 14.6/31, compared with a 12.0/26 for Four and an 11.0/23 for Seven. Channel 5's news at 10 p.m. averaged a 7.5/12 . . .

The Nielsen audience sample over the past couple of years tends to favor Nine. The Arbitron ratings, which were not available Friday, may draw a slightly different picture . . .

On the Other Hand

The American Film Institute has made an agreement with the three networks to televise its annual "Salute to . . . " specials on ABC, NBC and CBS on a rotating basis through 1991 . . .

On April 26 NBC will broadcast the "American Film Institute Salute to Billy Wilder," which will be taped March 6 in Hollywood. ABC will broadcast the 1987 Salute, CBS the 1988 and so on . . .

ABC was named Friday in a $1.5 billion lawsuit by movie studios and producers in a dispute over which side should get the right to claim federal income tax credits for shows aired on the network . . .

Seven studios and six leading independent producers -- comprising a veritable "Who's Who in Hollywood Production" -- accused ABC of "double-dipping" by trying to recover in U.S. Claims Court the federal investment tax credits allowed to producers . . .

The producers contend ABC told them since the 1970s that they would receive undisputed federal investment tax credits in return for accepting lower than usual television license fees, which the network pays to producers for rights to broadcast programs twice during a season . . .

But since 1982, ABC has been seeking to recover the same credits for itself in U.S. Claims Court, according to a statement from the Motion Picture Association of America . . .

NBC and CBS do not engage in such "double-dipping," according to the complaint, which was filed in Los Angeles Superior Court . . .

"We regret that the producers have apparently decided to file this suit, which is unfortunate and wholly without merit," said Patricia J. Matson, ABC vice president for corporate relations. "We continue to be willing to discuss the range of issues between us and the producers in good faith" . . .

Plaintiffs in the suit against Capital Cities/ABC Inc. include Columbia Pictures Industries Inc., MGM/UA Entertainment Co., Paramount Pictures Corp., 20th Century-Fox Film Corp., Unversal City Studios Inc., Warner Bros. Inc. and Walt Disney Co. Independent producers include Aaron Spelling, Lorimar, MTM, Steve Cannell, Witt Thomas Harris and Carsey-Werner . . .

NBC is trying another edition of "Fast Copy," which features stories and editors from such magazines as New York, Time, Esquire, TV Guide, Forbes and Playboy in a one-hour format . . .

Despite a very uneven performance (Captain Airwaves had a feeling the voice-over in most segments of the first broadcast didn't understand what he was reading), "Fast Copy's" debut on Dec. 27 finished 25th among 67 primetime programs rated the week ending Dec. 29 . . .

And Finally

A tip of the ole rabbit ears (appropriately muffled in blue, of course) to Channel 7 reporter Kathleen Matthews, who gave birth to 9-pound 8-ounce Thomas Wednesday at Georgetown University Hospital . . .

Proud father is Christopher J. Matthews, press spokesman for House Speaker Thomas P. (Tip) O'Neill Jr. . . .

Thomas joins 3 1/2-year-old Michael Kerry on the growing Matthews roll call . . .