There were no words on the cake, but to the musically literate the notes spelled out "Happy Birthday to You" in the key of F, even though the bar lines were out of place, seriously displacing the accents. Jean-Pierre Rampal picked up a knife, aided by his wife, Franc,oise-Anne, and with two hands on the knife handle, looking for all the world like a happy couple posing for a wedding picture, they made the first cut in the cake, carefully avoiding the sheet music that was its basic motif.

A gala crowd in the Watergate's Terrace Restaurant, raising champagne glasses, last night sang "Happy Birthday to You," the fourth time the world-famous flutist had heard the melody since 6 p.m. on his 60th birthday, which was yesterday and which he celebrated with the National Symphony Orchestra and other friends.

He heard it first shortly after 6, when he was ushered into the Grand Foyer of the Kennedy Center and welcomed by 75 flutists, members of the Flute Society of Washington, who played it in a special four-part arrangement in the style of the great baroque flutist, Kuhlau, and followed it with a serenade of works by Bach, Pachelbel, Telemann, Gabrieli and others -- most notably an arrangement of Rimsky-Korsakov's "Flight of the Bumblebee."

The second time was after intermission in the National Symphony Orchestra's concert, when an army of juvenile flutists marched on stage and played it again. The group included young Patrick Meisch, son of Luxembourg's Ambassador Adrien Meisch, who was making his Kennedy Center debut. When admirers commented on young Patrick's performance during the party at the Watergate, the ambassador (who is a professional-level pianist) beamed with paternal pride.

The third "Happy Birthday" came when Rampal walked into the restaurant after the NSO concert, where he played his own arrangement of Khachaturian's Violin Concerto (which loses something in the original) and a Bach sarabande as an encore. Rampal beamed at the musical greeting but said nothing until after he had cut the cake and heard the tune one more time. By then, he had received several of Mstislav Rostropovich's bear hugs and the less formidable homage of other admirers. Rostropovich also kissed and hugged Chief Justice Warren Burger and dozens of other friends.

But the NSO conductor's final gesture was verbal, not physical. "Jean-Pierrechick," he cried, using a Russian diminutive of affection to summon the flutist to cut the cake. "You know about magic flute," Rostropovich told the partygoers as Rampal approached the knife. "Jean-Pierre Rampal; that's magic flute. We have many good players on many instruments, but only one who makes revolution. That's Jean-Pierre Rampal." Then he reached for a marzipan flute that decorated the cake, just under the "Happy Birthday" music. He handed the flute to Rampal, who raised it to his lips with an expression of dismay as it bent out of shape in his hands. Rampal blew hard, produced a few notes (perhaps with the aid of a flutist hidden offstage) and then took a bite out of the candy instrument, which tasted better than it sounded.

The standing-room audience at the Kennedy Center was divided about evenly between black-tied partygoers and hard-core members of the Rampal fan club, who joined forces to give him a standing ovation at the end of the concert.

Among the musical connoisseurs was Martin Feinstein, whose wife, Bernice, complained during intermission, "I had to sit next to a man who conducted all through the Schumann symphony."

Hardest core of all were the members of the Flute Society, which is one of the oldest in the country and has some 300 members. The 75 who turned out for the pre-concert serenade were nearly twice as many as had been expected, and this led to some logistical problems. The National Symphony had offered standing room at the back of the sold-out concert hall for 40 flutists, but could not accommodate 75. "How many did you say?" an NSO official asked Jan Pompilo, president of the association. "We had 75," said Pompilo, "but some of them are gone." "We can find room for about 40 somewhere," said the NSO official, and he fit in about 43.

"He is our hero," said Pompilo. "We have 300 members -- the largest flute society in the country -- but we could not have gotten so many to play for anyone else."

National recognition was given to the flutist by President Ronald Reagan, who was represented at the party by his new national security adviser, William Clark, and the new chairman of the National Endowment for the Arts, Frank Hodsoll. In recognition of the birthday, Hodsoll presented Rampal with (no kidding!) a copy of this year's White House Christmas card, painted by Jamie Wyeth and signed by the president.

At the heart of the evening was music, beginning with the flute serenade and ending in the Watergate with Jackson Baine of Channel 5 and Paul Teare of WGMS-FM at the piano, playing their four-hand variations on "You Are My Sunshine" after most of the guests had left and the champagne had run out. Not to mention an NSO concert and the four renditions of "Happy Birthday" in between. But the moment that will last longest in the memories of those who were there is that of Rampal, alone on the stage, playing a Bach sarabande for people who love him, as beautifully as it can be played.