If Mark Twain was half the raconteur that Hal Holbrook is when he plays Mark Twain, the legendary author/lecturer must have been something to see. "Mark Twain Tonight!" -- the venerable granddaddy of one-man shows -- is as engaging an entertainment as ever.

Last night, Holbrook launched a five-performance run in the Kennedy Center Opera House, which ends Sunday before he goes off on another world tour. Playing the white-maned Samuel Clemens, who lectured widely under his nom de plume at the beginning of the century, Holbrook sported a courtly cream suit -- and rather less makeup than when he originated the character 31 years ago. Then he was in church basements, a budding actor of 29.

Last night, doddering incisively across the stage, exploding in rheumy cackles, waving his cigar like a wizard's wand and even nodding off to sleep, Holbrook exuded "the serene confidence of a Christian with four aces." And why not?

Although he might have been expected to tire of the character after all these years, he manages to convey a surprising spontaneity. And although the audience, by now, might have been expected to tire of his performance, they seemed to want more and more.

In Holbrook, Mark Twain is alive and well -- still joyously savaging the world's hypocrites, still sadly lamenting human folly, still savoring silly yarns. It's less a case of impersonation than possession.

Of course, the material can't miss -- it's faithfully drawn from Clemens' speeches, essays, novels and various autobiographical writings. They are as fresh today as ever -- and well worth repeating. An incomparable storyteller, Clemens was also no slouch in the one-liner department (and Holbrook's timing is honed to a fare thee well). To wit:

* "I used to tell lies but I've given it up. The field is overrun with amateurs."

* "All you need to succeed in medicine is ignorance and confidence."

* "I knew that money was the root of all evil -- I wanted all I could get."

"I wonder if God invented man because he was disappointed in the monkey."

* "Washington is a stud farm for every jackass in the country."

Holbrook, who doesn't like to be pinned down to a specific program, has under his belt some 12 hours of pure Twain to draw on. But he has more than that -- he has captured Twain's spirit.