Mark Twain is back in the Kennedy Center Opera House, pretending to be Hal Holbrook, unless it is the other way around.

In the 29 years Holbrook has been performing one version or another of his celebrated one-man show, "Mark Twain Tonight," the distinctions have been obliterated. So complete and masterly is the identification of the actor with his subject that it probably would take Twain's mother, or Holbrook's, to say which is which.

This is Holbrook's fourth engagement at the Kennedy Center (it runs through Sunday); no one is quite sure how many other local appearances he's chalked up. No matter. "Mark Twain" remains the best of the one-man shows, as alert and timely as if it had been conceived yesterday. Wandering through the author's writings, sometimes picking this selection or that among the dozens he is equipped to perform, Holbrook shapes a different show every night.

You usually can expect a chunk of "Huckleberry Finn," and it wouldn't be an evening of Twain without those fulminations--delivered less in anger than in pure amazement--against the baseness of the human race, the destructiveness of religion, the cupidity of Congress and the French, in general. His descriptions of the sun rising over the broad Mississippi are even more poignant in these environmentally imperiled times. And for sheer nonsense, there's no equaling "His Grandfather's Ram," a meandering saga that takes every possible tangent, resolutely refusing to come to the point. Indeed, so sure is Holbrook's control of the material that he dozes off at the end of that one, leaving the audience rapt while he snores.The celebrated makeup job used to take Holbrook 3 1/2 hours, although by now age may have allowed him to shave off a half hour or so. Dressed in a cream-colored suit, his eyes atwinkle, wafts of gray cigar smoke serving as his halo, the actor ambles on stage for the sort of lecture Twain himself was giving in his 70th year. The pace is studiously unhurried. But it also is artfully calculated.

The pause Holbrook takes to relight a cigar, or fumble with the stack of books on a table, does not spring from a moment's distraction, although it may look that way. Actually, Holbrook is setting us up for the comic kill. Like those paintings of hills beyond the hills beyond the hills, "Mark Twain Tonight" has punch lines beyond the punch lines beyond the punch lines. Just when you think he's come to the end of the line--he's gazing sweetly off into space, after all--another rejoinder suddenly springs from his furrowed brow.

Warm and sagacious, this surely is one of the American theater's most beloved characterizations. Unless, of course, it's Twain himself and no one's saying. I wouldn't put it past him, the imp.

MARK TWAIN TONIGHT. A one-man show with Hal Holbrook. At the Kennedy Center Opera House through Sunday.