Some realities are permanent, emphatically including those that are the stock-in-trade of comedian Sid Caesar. He was the guest artist at last night's National Symphony Orchestra pops concert in the Kennedy Center Concert Hall, and 35 years melted away.

It would be hard to name anything that has not changed since the early 1950s when Caesar was as well known as Johnny Carson today. But his dialogues and pantomimes focused on basic human realities, and his truths have remained essentially the same: the worries of a man walking up the aisle to get married; the gestures and attitudes of a married couple quarreling; the hesitations of a boy at his first dance; and the powerless, self-centered world of a 6-month-old infant.

Dealing in such hard-core realities, Sid Caesar has changed words and nuances in his routines to keep the material fresh. But he has not had to adapt any of it for the changing life styles of the '60s, '70s or '80s. When he recites a steamy passage from a Harlequin Romance first in English, then in his approximations of German, Italian and Japanese, time dissolves and we laugh as we (or our ancestors) did 35 years ago. When (in the persona of Professor Ludwig van Knowitall), he solemnly announces that "the main cause of divorce, unequivocally, is marriage," we smile at the tradition of such statements as much as the content of that particular sentence.

That's all very well, but one cannot help wondering: Why does he have a 100-piece orchestra sitting on the stage with him? In fact, for a while, it looked as though the National Symphony would have nothing to do but play a few bars of Mendelssohn's Wedding March or "Rock-a-Bye, Baby." But he had two routines that required an orchestra, and they were among the best of the evening.

In the first, Caesar and his associate Gerrianne Raphael mimed a married couple having an argument, with their gestures precisely choreographed to the first movement of Beethoven's Fifth Symphony. It was the kind of routine that has to be perfect to work at all, and it worked beautifully. In the second routine, Caesar sat at an imaginary piano and played the first movement of Grieg's Piano Concerto in synchronization with the actual playing of his music director, Elliot Finkel. Caesar injured the little finger of his left hand by snapping out a chord too vigorously, and momentarily disabled all his fingers when the invisible lid of his imaginary piano came crashing down on his hands. He also fiddled outrageously with the tempos, rhythm and phrasing, but Finkel synchronized the piano sound with him precisely.

Before intermission, guest conductor William McGlaughlin conducted capable interpretations of Dukas' Fanfare from "La Peri," Strauss' "Tales From the Vienna Woods" and three dances from Falla's "The Three-Cornered Hat." The program will be repeated tonight.