IF YOU WEREN'T invited to the Saudi ambassador's party-cum-fashion show a month ago, you can stop fretting. Twenty-three of the fabulous costumes that draped the models there are on display now at the Textile Museum in "Traditional Costumes of Saudi Arabia." Tr'es sheik.

Many of the costumes come from the collection of Princess Haifa, the ambassador's wife and the youngest daughter of the late King Faisal. It's been a dream of hers to gather all the old costumes and eventually start a museum for them in Saudi Arabia.

While the faces of women in Saudi Arabia may be veiled in public, the Textile Museum's dressmakers' mannequins have no faces. Standing on "dunes," they are protected from the heat of the day and the cool of the night with many layers of cloth. And to secure their modesty when sitting on the rugs or pillows that decorate the traditional home, they wear long, loose-flowing kaftans -- the thobe or thawb, worn by both men and women.

The thobe combined with a head covering: That's simple enough. But there is much more than meets the eye with traditional Saudi costumes.

From the central plain of the country, and home of the capital, Riyadh, come the most sumptuous dresses. The most elaborate of these are the wedding dresses -- worn by wedding guests, or by the bride herself, the day after her marriage at festivities given by the groom's family.

Made sometime after the establishment of Saudi Arabia in 1932, two wedding dresses on display here express subtle patriotism. On one, the palm and crossed-swords symbol of Saudi Arabia is a repeated pattern in dense sequins and gold thread on the dress and the hood. And on the other -- a diaphanous black thobe -- the national symbol is subtly woven and alternates with a railroad passenger car.

Most magnificent of all is the bridal costume itself, and this one comes from Medina near the western coast. Embroidered with gold thread, this rose silk chiffon fantasy from the Arabian Nights is coordinated to the fourth layer -- starting with a blouse and pantaloons (sidairya and sirwaal), which are covered by a thobe of the same material. Over it, on her chest the bride wears a pink pillow -- a yoke called a moukhada -- displaying family gifts of jewelry. And then the crowning glory, more jewels on a cloth tiara, and a flowing headcloth (guna'a) so encrusted with gold that the pink chiffon under it merely glows through.

This festive, sequined attire endures, says Johara Alatas, an information officer at the Saudi Embassy. "They continue to wear long thobes -- floral, made from whatever material they want, really -- but not as elaborate, and there's practically no handwork any more, apart from the ones with sequins."

The traditional costumes are now an unusual sight. Alatas says that at the party, an older gentleman, who was part of a folkloric troop from Saudi Arabia, burst into tears at the sight of a model dressed in a shroud-like outer garment, her face covered with the traditional starched white mask.

Says Alatas, "She reminded him of his mother." TRADITIONAL COSTUMES OF SAUDI ARABIA -- At the Textile Museum through May 3.