Celebrating the 143rd birthday of a theater on its site, the National last night had reason to feel smug. Its auditorium and stage fit the musical smash of the '70s, "A Chorus Line," far better than the grand, vast Kennedy Center Opera House, where another company played last season.

This is good news because the company will be playing for at least three months in the refurbished, 1,680-seat theater. If there ever was any question about the National's viability in today's world, its neat fit for "A Chorus Line" is one more handy answer.

Coming out in the mid-'70s, "A Chorus Line" benefits from brilliant timing. Our period is insatiably curious about the most ordinary people, seeking out and defending individualities who in an earlier era would be swept under the carpet. Being concerned with how a handful of dancers will be winnowed from many to make the anonymity of any old chorus line, the series of stories developed from dancers themselves, is strikingly suited to our democratic times.

And, with Michael Bennett's conception, choreography and direction, "A Chorus Line" falls into a perfect time slot, a time when, after such folk as Sol Hurok, Lucia Chase, Lincoln Kerstin and other trailblazers had labored in the vineyards, dance captiivated America. By 1975 we were ready for exactly this kind of musical.

Created in "workshop" sessions, a form which seems to be catching on, "A Chorus Line" also benefits from striking fusion of details. Tharon Musser's striking lighting scheme for Robin Wagner's mirrored setting is stage lighting of peerless skill. The Marvin Hamlisch score has its period's nervous intensity, reiterated rhythms, jagged melodies, and the book by James Kirkwood and Nicholas Dante opens up variations for lyrics at which Edward Kleban has proved highly adept. Tom Hancock's musical direction, on which five musicians were early collaborators, will suit your recollections of the recording.

One trick of involvement with the two-hour adventure is to pick out dancers and see whether they make Zach's final choices. Once again, I lost out and felt that Rita O'Connor's Sheila and Jeffory Robinson's Al would have embelished any line. But then, Anthony S. Teague, the Zach, couldn't change the plot.

Wanda Richet's Cassie, who inherits "The Music and the Mirror" number, does it with high passion, and there are other concise vignettes from all.

Looking up and back at the packed, celebrity-littered house, one longtime National patron reminisced:

"To sit in the top balcony, the cheapest seats, we'd line up in the street. I was a little girl, only a bit smaller than I am now. When the door opened, I'd dash up the three flights of stairs, squirming between people's legs to get there first. I'd literally lie across two seats in the first row and Mother would find me. My, that was over 70 years ago. Doesn't the new red color look nice? It's all the same, only different."

Her name was Helen Hayes.