They threw a cozy party tonight at the Carlyle Hotel to celebrate the 20th anniversary of Washington's International Exhibitions Foundation. The IEF -- one of the best known secrets in the art world -- is really Annemarie H. Pope. In two decades of ceaseless and at times ferocious labor, Pope has organized more than 150 exhibitions. Her nonprofit foundation is in part a booking agency, and in part an international museum without walls.

The black-tie dinner was attended by a wide assortment of curators, collectors, museum directors and artists. One by one her debtors rose to give their thanks.

"Your grace, my lord, baron, excellencies, distinguished guests -- am I getting the hang of it?" asked National Gallery Director J. Carter Brown. "I would like to put into your heads a sentiment paraphrased succinctly by the Duchess of Devonshire. Ask not what you can do for Annemarie. Ask what Annemarie can do for you."

In the last 20 years, Pope has organized 25 exhibitions for the National Gallery of Art. "When I first took my job," said Brown, "she was it, she was our exhibitions department."

"How does she do it?" asked David Lloyd Kreeger, her foundation's president. "She has no endowment. She runs no fund-raising drives. I'll tell you how she does it, she does it with mirrors."

"You are a formidable lady," said Simon de Pury, Baron Thyssen-Bornemisza's curator. "You are also very charming. I think of you as iron in velvet."

She often sups in castles. Millionaires accept her. Tonight, accompanied by nobles (the Duchess of Devonshire, the Marquis of Hartington and Baron Thyssen-Bornemisza, all three of whom have worked for her as IEF trustees), she might have been mistaken for a European countess. But there is another side to Annemarie Pope. To see her in her office at 1700 Pennsylvania Ave. NW, pleading on the phone with lenders round the world, discussing shipping and insurance and catalogue production, one might instead regard her as a workaholic drudge. Since her late husband, the courtly John A. Pope, a former director of the Freer Gallery of Art, died in 1982, she has labored, tirelessly and ruthlessly, seven days a week.

Millions of Americans have seen her exhibitions, though they may not have known it. Museums, far too often, have gladly taken credit for shows she has arranged. Last year's awesome showing of "Old Master Drawings From the Albertina" -- she began angling for its loans in 1968 -- was an IEF exhibit. So were previous displays -- in galleries in Washington -- of works by David Hockney, Ingres, Edouard Vuillard, Milton Avery, Gainsborough and many other masters of the graphic arts.

"We do not lend our works of art to American institutions," said the Marquess of Huntington. "We lend them to Annemarie." His parents, the Duke and Duchess of Devonshire, have circulated four shows through Pope's foundation -- and the fifth is on its way.

The guest of honor, dressed in pearls and black and gold, recalled, "The first one was circulated during the Kennedy administration. The largest -- 'Treasures From Chatsworth' -- was the seed which led to the present exhibition at the National Gallery. We had one house, now they have 200."

"I'll tell you how she does it," said Muffie Brandon. "She never forgets her debts. Late at night, she sits there at her desk, beneath a little green globe, and writes her thanks to everyone. She first demands and she charms. She simply wears them down."