Any time Leonard Bernstein introduces a new composition of his on a program, you would expect it to be the highlight, but in last night's concert with the National Symphony he proceeded to steal the show from himself with a blazing performance later on that was what people went away remembering.

That closing work was Elgar's "Enigma" Variations, the set of 13 portraits of his closest friends and associates and one of himself that constitutes the most eloquent set of musical tributes ever written. It is not a piece normally identified with Bernstein, and that in itself may help explain why the Variations tapped the emotional reserves usually identified with the younger, and more impetuous, Bernstein to an extent that I haven't heard from him in some years.

It's hard to explain, but I think the key is genuine excitement on the part of Bernstein. The result is a freshness and passion, an extroverted way of making music, that sweeps everything in its path when properly controlled.

In recent years as Bernstein has leaned toward more inward interpretations and concentration on refinements, he has sometimes sounded like he was rejecting his former self. After last night's Elgar you can forget that worry.

Some of the tempos are very slow (and what's new about that?) and will take some getting used to. The "Nimrod" variation, which is dedicated to Elgar's closest friend and is the work's centerpiece, was played as slowly and intensely as Bernstein will play a Mahler slow movement, and it was just as effective.

Bernstein's new work is a tribute, too. It is called "Halil," Nocturne for Solo Flute, String Orchestra and Percussion. Halil is the Hebrew word for flute, and the work is a memorial to Yadin Tanenbaum, a 19-year-old Israeli flutist killed in 1973 in his tank in the Sinai. Bernstein says he never met Tanenbaum, "but I know his spirit."

Whatever, "Halil," first played last fall with the Israel Philharmonic and Jean-Pierre Rampal, is a contemplative, moving work of about 15 minutes. It switches from the atonal, identified with war and death, to the tonal, identified with peace and, finally, says Bernstein, with "sleep itself, Death's Twin Brother."

Of Bernstein's reflective pieces, it is not one of his major statements--less deeply felt than many. The solo flutist was the New York Philharmonic's legendary Julius Baker, his sound at its most sumptuous.

The Philharmonic's solo violist, Sol Greitzer, performed in Walton's Viola Concerto, a cornerstone of the slender viola concerto repertory that was played very well in honor of the Englishman's 80th birthday. The National Symphony played at its very best.

The program will be repeated tomorrow and Friday nights.