"I used to know all the Fats Waller songs," said former Rep. James W. Symington (D-Mo.) last night in the lobby of the Warner Theater between acts of "Ain't Misbehavin'." "I even painstakingly learned 'Ain't Misbehavin' ' in a simplified form. I used to play it at all the parties I went to. People learned quickly. Soon there were no requests."

So at the party at the British Embassy, given by the board of the Foreign Students Service Council, after the play, Symington was in fine form. When J. Leonard Oxley, the conductor and pianist for the play, strode quietly onto the terrace, Symington approached him eagerly.

"Do you remember," the former congressman and chief of protocol asked Oxley, "a Fats Waller song about a little birdie on a tree? Little-bird-ie-on-a-tree . . ." Symington snapped his fingers to the rhythm, and Oxley nodded vaguely in remembrance.

Almost all of the 380 guests who came to the benefit preview of the Fats Waller musical raved about the music. The Foreign Students Service Council filled one-fourth of the theater with benefit contributors. This was their annual fund-raising effort. Last year they held their fund-raiser at Meridian House, and the talented Symington, who is this year's chairman of the council, sang and played guitar.

"This is my period," said former CIA director Richard Helms during intermission. "I used to go to the Cotton Club and enjoy all his music."

During intermission, lawyer and businessman Alan Novak grasped the hand of Mrs. Potter Stewart, wife of the Supreme Court Justice, and inquired, "Where is the great man?"

"Well, unfortunately the great man isn't enjoying this as much as I am," said a bubbly Mrs. Stewart to Novak, one of the justice's former law clerks. "He's almost asleep right now in his seat. It's just been such a long day."

Many of those who stayed awake made it to the elegant hall of the British Embassy where Margaret Jay, wife of outgoing British Ambassador Peter Jay, hosted a supper party.

The council, which extends a helping hand to foreign students living here or traveling through and also sponsors regular seminars for international graduate students, pulls its members from government, foreign service work, and those who have simply traveled abroad. One of the council's former chairman, Joe Palmer, was ambassador to Nigeria and Libya at different times.

"We keep collecting foreign students," said Palmer's wife Margaret at the party last night. "Whenever the council needs people to put up foreign students, we always ask, 'Can we please have the Africans?' And they say, 'Well, we have to spread them around.' "

Betsy Brown, the council's executive director, arranged for the benefit performance of "Ain't Misbehavin'." And benefit chairman Carol Wakelin considered it a success (it raised $17,000) during a tough week to have social successes. "When we started working on this, I thought 'We'll never get anyone to come.' But I sold my last ticket today." About 70 to 80 of the tickets bought were donated to foreign students.