The pink and green balloons were fluttering in the afternoon breeze yesterday in the courtyard of the National Museum of American Art. The lollipops and lawn chairs and pink tablecloths were in place. And when the surprise guest of honor appeared and the crowd of well-wishers began a chorus of "Happy Birthday," a smile as bright as the sun crossed her face.

Adelyn Dohme Breeskin, the museum's senior curatorial adviser, will be 90 on Saturday, and about 100 of her friends from the art world were determined to make the most of it.

"She is ageless and just as vital now as she was then," said museum docent Loretta Rosenthal, who has known Breeskin for a number of years. "We feel that she's our mom of the arts."

The affectionate title seems appropriate. The white-haired woman with the silver-tipped cane began her career in 1918 working in the print rooms at the Metropolitan Museum of Art. From there she moved to the Baltimore Museum of Art, where in 1947 she became the first woman to direct a major museum. She helped to found the Washington Gallery of Contemporary Art in the early '60s and later moved to the National Collection of Fine Arts. Since 1968 she has been a curator at the National Museum of American Art. A renowned Mary Cassatt specialist, she is working on a new edition of her 1970 landmark catalogue of Cassatt's paintings and drawings.

It was a celebration of hugs and kisses and birthday gifts that included a floral centerpiece, a book on art in France, and a cake bearing a fleur-de-lis design from the cover of one of Breeskin's books on Cassatt. "Happy Birthday, Mrs. B." read the hand-iced salutations, straight from the kitchen of museum docent Sophie Danish. There was even a special birthday album of greetings from friends here and abroad.

But the pie ce de re'sistance was the unveiling of two works purchased from a special fund in honor of Breeskin to be donated to the National Museum: a large painting by Jacob Kainen titled "Dabrowsky V" and a wood sculpture, "Black and White Tipped Flower," by James Surls. Several other works were given to the museum in her name.

Looking happy and moved, Breeskin sat at one of the tables with the album beside her and listened as birthday tributes were offered by museum Director Charles C. Eldredge, Daniel Terra, the State Department's ambassador at large for cultural affairs, and Smithsonian Assistant Secretary Tom Freudenheim.

In his speech, Terra recalled dining out with Breeskin. Sometimes, he said, she would drive, and "I've never seen anybody jump in the front seat of a car and flatten the accelerator like she did."

After the ceremonies, the more formal guests lined up as though in a receiving line for a queen. Others ignored protocol and clustered around her. Repeatedly chairs were offered for Breeskin to sit, but she preferred to stand, greeting artists, collectors and moving forces of the art world.

"Bless your heart, dear. Bless your heart," said lawyer Rufus King, pulling a white lawn chair for for her to sit down in. "Let's make them come to you." But the guest of honor remained standing.

Charles Parkhurst, who succeeded Breeskin as director of the Baltimore Museum, began to reminisce about Breeskin's days there. "She had a chair behind her desk, a tapestry chair, which nobody else could sit in because she looked like a queen because of her nobility and bearing. No chair overwhelmed her."

It was a day for fond and admiring looks back. Tom Beale, chairman of the Hereward Lester Cooke Foundation, which awards grants for artists in midcareer, recalled, "My favorite story about Adelyn is that she headed the committee for the Cooke Foundation. Now one day she reviewed 1,000 slides over an eight-hour period. The next day when she came to review the second thousand, she opened by saying, 'Last night when I got home I was mentally reviewing the slides, and I think we did one artist an injustice in dropping him from further consideration.' We pulled the slides. His work was reviewed again. He became a finalist and in the end was awarded a grant."

Standing not far from a fountain surrounded by artworks, including a whimsical Calder sculpture, Breeskin turned reflective. "I feel very strongly that art was what I was supposed to concentrate on. And I've done it."

The surprise party was the icing on her birthday cake. This week Breeskin is leaving with her daughters for two weeks in Europe, and her plans for the trip -- and her future -- came as no surprise: "More good fun and art . . . I'm going to see beautiful sculpture, romanesque art. We've had so much interest in Gothic art, but romanesque is even greater."

The party was just beginning to break up. Guests still milled about, chatting and enjoying the refreshments, and Adelyn Breeskin stood taking it all in.

"I'm shoving off," said Parkhurst, giving her a bear hug. "Here's to your 91st."