The faces American pop culture - and a few immortal ringers - stare across the empty mezzanine above the National Portrait Gallery's third floor.

The new show, opening today, is the second installment of Time magazine covers acquired by the Gallery last year. The first 100 were such a success when they were shown here that they are now touring the country.

This show concentrates on the faces of art and entertainment. They range from a 1933 portrait of George M. Cohan to a 1977 photo of Diane Keaton.

There are no strangers here. These are the faces we have been brought up on and spent our evenings and Saturday afternoons with, people who require only one name: Streisand. Nureyev. Faulkner. Cavett.

The early ones are mostly movie stars, members of the generation that came after Cohan, who brought back Broadway during the Depression. There is a boyish John Wayne, painted in 1952 by Boris Chaliapin, the cover artist who became almost as famous as his subjects. Wayne was only 44 then and the background depicts a showdown on a Western street and a cash register.

Close to him are Kim Novak in 1957, Anne Bancroft in '59 as Annie Sullivan, and a striking Chaliapin portrait of Lauren Becall.

There are two covers of Mia Farrow, both with men and both heralding films that flopped. In her '69 picture with Dustin Hoffman, "John and Mary," she looks merely winsome, but five years later with Robert Redford in "The Great Gatsby," she has that eerie combination of the palpable and mythical, the sensuous and ethereal, which Gatsby's lost Daisy must have.

In later years, of course, we see TV coming on the scene. The earliest probably is Robert Berks' 1967 bust of Johnny Carson, and the latest must be Carroll O'Connor as Old Man Winter in a 1973 comment on the first energy crisis. Peter Falk is there too, in his immemorial raincoat.

Among the much-promoted figures are some who never needed the likes of Time magazine to make it.

Solzhenitsyn glowers from under the most famous forehead wrinkle of our time. Close by is Arthur Rubinstein, and near him are Zubin Mehta and George Solti. Beyond them are Tebaldi and Callas, two of the era's greatest ovices. And Toscanini, at 67, in 1934 - was he ever young?

Changes in styles are almost as interesting as the changes in our notions physical beauty. The early covers tend to be straight portraits, many of them byChaliapin and Henry Koerner, most strictly realist. The Bernard Safran painting of Tennessee Williams is virtually photorealist, in fact.

Aside from a large, swirly impression of Nureyev by Sidney Nolan and perhaps Chaliapin's slight pencil drawing of Sandy Dennis, which perfectly captures her essential wispiness (not to say wistfulness) there isn't much experimentation here.

Later we see oil-and-photo collages of George C. Scott and Andy Warhol's delightful comment on the Flying Fondas, a cut-paper likeness of Elliott Gould and a 1972 Flip Wilson surrounded by the lights of a dressing-room mirror.

This is one exhibit where you won't have to work. It's just for fun. And when you're done looking, remember, there's plenty more where they came from. The collection comprises almost 900 covers. After all, Time has been going on since Henry Luce got out of college in 1923, and did you ever see a picture of him that looked young? CAPTION: Picture, "The Flying Fondas," Time cover by Andy Warhol, 1970.