Princess Grace had been signed to narrate it by her old friend Mstislav Rostropovich. Then, with her death in September, it seemed the National Symphony Orchestra's March 28 Pension Fund Concert performance of Camille Saint-Sae ns' "Carnival of the Animals" was unlikely to happen.

Now, in the grand old tradition of the show going on, another former Hollywood actress, who once trod the same Metro-Goldwyn-Mayer lot as the former Grace Kelly, has come to the rescue. Nancy Reagan has agreed to a one-night comeback, in front of the Kennedy Center Concert Hall footlights. She'll narrate verses of Ogden Nash that are traditionally used with the Saint-Sae ns music, demonstrating "her esteem for her friend Princess Grace and her commitment to the arts in America," says the White House.

With a "cowboy" in the wings and a radio in the stall, what more could a gift horse ask?

In the case of Amadeus, the Austrian Lipizzan presented to the First Cowboy last fall, probably not much. Put a similar question to the "cowboy"--Ronald Reagan--and the answer might be, "a little dressage, thank you, but definitely no more gift horses."

The White House says Amadeus is alive, well and happy at a privately owned Sykesville, Md., farm but that he did not, does not and will not belong to the president--despite the good intentions of the Austrian Chamber of Commerce.

The Austrians say Amadeus, officially known in Vienna's famed Spanish Riding School as Maestoso Blanca, is of "inestimable" value, which is another way of saying the 12-year-old stallion can't be bought. That makes him priceless, and since priceless gifts cost more than the $140 limit set by Congress, it was illegal for the president to accept him.

But riding Amaedus isn't illegal. With that end in mind, some of the president's friends pitched in to find the $200-to-$300 a month it takes to keep Amadeus in the style to which a priceless Lipizzan is accustomed.

It wasn't terribly difficult. A senior White House official, who declined to be identified, said the White House received letters from breeders and associations around the country, including Tempel Farms in Wadsworth, Ill., whose own herd of 400 Lipizzans is the largest under private ownership in the world.

"It was the intention of the Austrian Chamber of Commerce to keep the horse in this area and have it ridden by the president. They understood that by reason of our severe rules, title should remain theirs," said the official. "We felt a responsibility to the chamber to see that the horse receive the best care, feeding and exercise."

For the First Cowboy, said the official, "it was love at first sight, but he was unable to ride because of the quarantine rules."

For some in the White House, though, the horse had all the thrill of a diplomatic headache. By proliferating the idea that the president likes to ride, it became a kind of one-upmanship among countries.

"Sure enough," said one official, "the president goes to South America and a chief of state Brazil gives him a horse. Now we have two horses the president can't accept."

(A Brazilian Embassy spokesman says President Joao Figueiredo knew in advance that his gift horse couldn't be accepted, so he is merely sending it to Washington for Reagan to ride.)

Meanwhile, back at the farm and oblivious to all this, Amadeus chews his mixed timothy hay, his specially formulated feed and his natural vitamins under the watchful eye of Dedication Farms' owner Helena Amis Clifford. When he isn't being exercised, he's lazing about in his private stall listening to his private radio.

Already a celebrity with more boarding offers (including some from horsey Middleburg) than he knows what to do with, Amadeus' future after quarantine is still undetermined. But that may not be the case for long.

"The president is looking forward to the weather changing," says the senior official, one of the few who has test-ridden Amadeus. "Come the blossoms, and you'll see the president out there."