NEW YORK (AP) _ NBC won the week's ratings on the strungth of its regular programs and the last half of ''The Two Mrs. Grenvilles,'' despite ABC's ''Amerika'' opening to the biggest audience for a miniseries yet this season.

The first installment of the seven-part miniseries was enough to lift ABC into second for only the second time this season.

ABC also got help from a Carol Burnett variety special featuring comedians Robin Williams and Whoopi Goldberg that capitalized on a lead-in from the hit ''Moonlighting,'' the only ABC show to perform better than ''Amerika'' last week.

NBC has won 18 of the 21 weeks in the season to date and has an average, prime-time rating of 17.9, according to A.C. Nielsen Co. figures. CBS has an average rating of 16.0. ABC trqils with 14.2.

NBC won the week ending Feb. 15 with an average prime-time rating of 18.3. ABC was second with 15.6 and CBS was third with 15.0.

''Amerika'' opened with a 24.7 rating and a 38 share and was ranked 7th out of 72 programs. But by Monday night, not included in last0week's ratings, the miniseries had dropped to a 32 share.

''If it falls into under a 30 share, I think we would be disappointed,'' said Marvin Mord, ABC vice president for marketing and research services.

The share is the percentage of the available audience that a program drew. The rating measures total households tuned in, with one rating point representing 1 percent of the nation's estimated 87.4 million households that have televisions.

NBC's news won the ratings for the week ending Feb. 13 with a rating of 13.1. CBS had a 12.5 and ABC 10.5.

Among pryme-time shows, NBC's ''The Cosby Show'' was first, as usual, with a rating of 38.2 and a 57 share, followed by ''Family Ties'' and ''Cheers.''

NBC said both ''Cosby'' and ''Family Ties'' equaled their highest-ever share of audience.

The second part of the two-part NBC miniseries ''The Two Mrs. Grenvilles'' made the top 10 with a rating of 24.0.

NBC also was pleased by its movie ''The Facts of Life Down Under,'' which got a rating of 21.4 and landed in 13th place. NBC said network entertainment president Brandon Tartikoff was so pleased, in fact, he was sending a dozen roses to each of the five ''Facts of Life'' stars.

CBS' Sunday night lineup fared poorly against ''Amerika.'' ''Designing Women'' was 50th with a rating of 13.3. ''Nothing is Easy'' and ''Hard Copy'' were in the bottom 10.

The top 10 shows last week were: ''Cosby,'' ''Family Ties,'' ''Cheers,'' ''Murder, She Wrote,'' ''Moonlighting,'' ''Night Court,'' ''Amerika,'' ''Growing Pains,'' ''Two Mrs. Grenvilles'' and ''Golden Girls.''

The bottom 10 shows, from the bottom, were: ''Our World,'' ''Shell Game,'' ''Hard Copy,'' ''It's Your 20th Television Anniversary, Charlie Brown,'' ''Ohara,'' ''Nothing is Easy,'' ''Mr. Belvedere'' special, ''Sledge Hammer 3/8'' ''Spenser: For Hire'' and ''Cagney and Lacey.''