For the first time since A.C. Nielsen Co. began keeping such figures 26 years ago, NBC has won a primetime season outright, after a 30-week campaign that began Sept. 23, 1985, and ended Sunday . . .

NBC tied CBS for first in the 1969-70 and 1970-71 seasons . . .

For the season, NBC had a 17.5 rating (representing an average of 15.0 million TV homes a night) and a 27 percent share of the nightly audience. CBS, which had been first six years in a row, dropped to second with a 16.7/26 (14.3 million) while ABC had a 14.9/23 (12.7 million) . . .

NBC was up 8 percent from last year, CBS was down 1 percent (although its households-delivered total of 14.3 million was unchanged) and ABC off 3 . . .

Tiptoeing through a blizzard of year-end statistics, here are a few of the high- and lowlights:

The top 10 regularly scheduled series for the season (with last year's ranking in parentheses) included NBC's "The Cosby Show" (3); NBC's "Family Ties" (5); CBS' "Murder, She Wrote" (9); CBS' "60 Minutes" (4); NBC's "Cheers" (12); CBS' "Dallas" (2); ABC's "Dynasty" (1) and NBC's new "The Golden Girls," tied for seventh; NBC's "Miami Vice" (44); and ABC's "Who's the Boss?" (37) . . .

The Really Final Five in l985-86 included NBC's new "Fathers & Sons" in 92nd place, followed, in order, by CBS' "Hometown"; ABC's "Our Family Honor"; ABC's "Fall Guy"; and ABC's "Shadow Chasers," which somehow managed to average a 5.8 rating and a 9 share to finish 96th and last. . .

The top five theatrical movies, in order, were ABC's "Mr. Mom," NBC's "First Blood," NBC's "48 Hrs.," NBC's "An Officer and a Gentleman" and a rerun of "48 Hrs."

ABC's "North & South" topped the list of mini-series, followed by four CBS offerings: "Kane & Abel," "If Tomorrow Comes," "Sins" and "Dream West" . . .

In the matter of made-for-TV movies, NBC's "Return to Mayberry" was first, followed by NBC's "Perry Mason Returns" and three CBS shows: "Intimate Strangers," "Nobody's Child" and "Rockabye" . . .

Among entertainment specials, the Academy Awards show on ABC was first, NBC's Bob Hope Christmas special was second, followed by three CBS shows: "All Star Party: Dutch Reagan" and, tied for fourth, "Circus of the Stars" and "The Grand Ole Opry's 60th Anniversary" . . .

Naturally, both CBS and ABC saw something promising in the wreckage . . .

CBS research chief David Poltrack pointed out that his network increased the number of adults 25-to-54 this season and ended a six-year trend, despite the previous wins, of lower audiences each season . . .

"People are coming back to the networks, and CBS benefited from that," said Poltrack . . .

CBS also won Monday, Friday and Sunday this year "by significant margins." NBC took Thursday and Saturday . . .

CBS has been dogged with poor ratings at the top of each night, except for Sunday, all year long. Poltrack pointed out that CBS lags by three ratings points at the start of primetime each evening but that from 9 to 11 p.m. each week CBS is first . . .

"And if you took out '60 Minutes' it would be four ratings points," he added . . .

He said CBS currently has 17 programs in development aimed at the young audiences that watch at 8 p.m. "Our rating among children is 5.8, while NBC's is 10.0" . . .

ABC can point to a strengthening "Colbys" on Thursday night and "The Disney Movie" on Sunday for hints of better things to come. ABC was also dogged by a disastrous year in TV movies, which should improve . . .

And both CBS and ABC are probably targeting Saturday night as a potential weak spot on NBC's schedule next season . . . while hoping NBC's "Miami Vice" fades and "Hill Street Blues" turns out to be on for one season too many . . . Also in the News

Jerry Fedell, who has been executive producer of Channel 9's 11 p.m. news for three years, has resigned to become news director at WKBW in Buffalo . . .

"Paris: A Tale of Two Capitals," which was supposed to air last Sunday on WDVM but lost out to the rescheduled conclusion of "Dream West," will air at 8 p.m. this Sunday instead . . .

Bruce Johnson and a WDVM camera crew were recently in the City of Lights preparing the documentary . . . It'll be repeated on May 2 at 11:30 p.m. . . .

Monday night, a repeat of "Something About Amelia" on ABC drew a national Nielsen rating of 12 and a 20 percent audience share, while the "TV Academy Hall of Fame" on NBC managed a mere 7.8/12 . . .

Why the unusually low network numbers? Because dozens of network affiliates around the country were carrying that syndicated "The Mystery of Al Capone's Vaults," that's why . . .

By now you may have heard that when they swung open the doors, "Vaults" host Geraldo Rivera found only a couple of old whiskey bottles . . .

In Washington, the "live" special was delayed until 11 p.m. after the Caps-Rangers hockey game on Channel 20 . . .

The two-hour special averaged a mere 4.3 rating and a 14 share on WDCA. One possible reason: The story of the empty vault was used as the 10:55 p.m. "kicker" on Channel 5's newscast Monday night . . .

Speaking of WDCA, "Notorious" was yanked without notice from last Saturday night's lineup when programmers discovered late last week that their barter deal for the movie "The Chosen" had a deadline that was suddenly up . . .

"Notorious" will be scheduled again sometime in the summer, according to a WDCA spokesman . . .

We hear that Jack Laurence will return to the ABC London bureau soon after finishing the "20/20" piece he's currently working on . . .

He and Lynn Sherr were brought aboard for four-month tryouts as possible replacements for Geraldo Rivera and Sylvia Chase . . . Sherr has now joined the "20/20" team permanently but the current budget problems at ABC News may eliminate the other correspondent position . . .