SHOWBOAT -- At Wolf Trap through June 1; at the National June 3 through July 6.

It's only the book of "Showboat" that can be considered a revival, because the songs haven't been out of the American popular repertoire since Jerome Kern and Oscar Hammerstein II wrote them in 1927. The full three-hour show, now at Wolf Trap and moving to the National Theater on June 3, supplies the original context of "Ol' Man River," "Can't Help Lovin' Dat Man," "Only Make Believe," "Life Upon the Wicked State," "Goodbye My Lady Love" and "Bill" (with lyrics by P. G. Wodehouse).

It serves as an excuse for doing that marvelous score, and as living theatrical history. There are dramatically touching moments. The singing by Lainie Kazan, Robert Mosley and Pamela Kalt, and the comic acting by Eddie Bracken and D'Jamin Bartlett are full and vigorous.

But a certain amount of toleration is required, because "Showboat" doesn't represent the perfect integration of story, dance and song as developed in the great musical comedies of later decades.

The Edna Ferber novel on which it's based goes from the 1880s to the 1920s, and deals with complex racial and emotional issues. These are merely sketched in the show's book, leaving chronology problems and loose ends all over the stage. By the show's end, the budding daughter, handsome young parents and peppy grandparents have to be, respectively, in middle, advanced and extreme old age.

The famous love story of the quadron and her white husband who tastes her blood to avoid arrest for miscegenation is never satisfactorily explained. We see her in Chicago in 1904, as a nightclub star who drinks too much, but there's no explanation of whether personal or social pressures finally killed the marriage. It's particularly puzzling that the place where she works is shown as having a comfortably mixed clientele of black and white rich people.

And the show's most famous song has no plot buildup at all. The character who sings "Ol' Man River" has never experienced the excruciating labor so eloquently presented in the song: by his wife's testimony, he hasn't moved a muscle in 40 years.

These notations are not really offered as complaints, because "Showboat" is important enough for its music to be worth doing as originally written. But at a time when an anthology of American musical comedy hits is being offered on Broadway -- "Oklahoma!" "Peter Pan," "West Side Story" and the pastiche "American Dance Machine" are there now, with other revivals coming soon -- it's well to remember that "Showboat" was a precursor.