Nancy Reagan will donate some of her wardrobe to American museums, the first lady's press secretary, Sheila Tate, said yesterday. Some of the clothes were recently "loaned" to the first lady from designers, who were advised about the project and knew their clothes would become part of costume collections in museums across the country.

The museum project is scheduled to be announced officially tonight at an awards dinner of the Council of Fashion Designers of America, to be held in the Great Hall in the main New York Public Library. Ann Keagy, the chairwoman of the Parsons School of Design fashion design department, will administer the project. She is expected to reveal the details from a telegram sent to the group by Nancy Reagan.

"This is being done to encourage the American fashion industry," said Tate, "which she [Nancy Reagan] feels is the best in the world. We really are rather pleased with this. Its significance is both historical and educational."

Tate said the project was not a way to avoid declaring such clothes as gifts. She said Mrs. Reagan plans to file a White House report in May, which must include the loaned clothes she has decided to keep. Another alternative, Tate said, is to pay for them.

Nancy Reagan has been an avid collector of top American designers, most notably Bill Blass, James Galanos and Adolfo. For last year's inaugural gala at the Capital Centre, she wore a Bill Blass black velvet and satin gown, estimated at $1,800. For the royal wedding of Prince Charles last summer, she wore a peach coat costume by Galanos, with a price tag of approximately $1,500. Her Chanel-style suits, which are almost her uniform for day, are by Adolfo. They retail for approximately $1,200 each.

But it was unclear whether any of these are en route to museums, because Tate said Mrs. Reagan hasn't decided which clothes to part with. However, her Galanos inaugural gown, loaned to her by the designer, was turned over last December to the Smithsonian Museum of American History's first ladies exhibit.

According to Bill Blass, Nancy Reagan has been billed until now as one of his private customers. "Several months ago, we discussed this idea of loaning clothes, which I think is marvelous," said Blass. Blass said he only learned that the project was under way when Mrs. Reagan's telegram was revealed at a planning meeting of the CFDA in New York yesterday.

"It's a fascinating idea, and I certainly hope that something will go to a museum in Indiana, my home state," he said.

Last summer, there was a fuss when Mrs. Reagan was spotted wearing borrowed Bulgari jewelry. Under an arrangement with the leading Italian jewelry firm, she chose gems -- according to a spokesman -- that were "hers to wear as long as she wants to wear them."

The clothes project is expected to be administered by Keagy, who will decide which of about a dozen museums receive which clothes. She will function as a volunteer and not as a representative of Parsons. Keagy said that some previous donors of clothes to museums have used them as tax deductions.

Blass, asked whether he or Nancy Reagan would get the tax advantage, replied: "That never really occurred to me."