The seemingly indefatigable Mstislav Rostropovich, who has toured three continents since he last came here in May. has run out of steam in Rio de Janeiro.

With only four days notice, the National Symphony conductor has cancel his two-night engagement scheduled to launch the orchestra's expanded new summer series, which starts this weekend at Wolf Trap.

Rostropovich called from Rio, where he had performed in seven concerts in seven days, to say that doctors have advised that he delay his return here for a week's rest because he is "overheated, exhausted and suffering from low blood pressure." The Friday opening concert is already sold out and the Saturday night program nearly so.

The news sent the orchestra management on a frantic search for replacement conductors. But by yesterday they had been turned down by Leonard Dernstein, Seiju Oawe, Sir George Solit, Andre Previn, Zubin Mehta, Erich Leinsdorf, Lorin Maazel and James Levine.

The man who finally agreed to lauch the series Friday night is Varoujan Kodjian, associate conductor of the Seattle Symphony. John Nelson of the Indianapolis Symphony was signed for Saturday night. The symphony said that neither man was high on its "shopping list."

According to Robert Noerr, manager of the symphony, the only big-name conductor who considered the replacement offers was Rafael Fruhbeck de Burgos, who finally declined on the grounds he had already conducted five times in the past seven days, I agreed," Noerr said. "The last thing we needed was another basket case."

Noerr blamed Rostropovich's exhaustion on the very strenuous schedule he has followed since May.

"You know it's most unusual indeed for an orchestra's music director to cancel out under such circumstances. It may be a factor of the jet age. They accept too many engagments, and it finally caught up with him. There is no way to chastize him for it. He simply gave out."

Rostropovich, who is 51, is reported not seriously ill. He has not been hospitalized, and his precise whereabout was unknown last night. Sources in Brazil said he was likely to have retreated for rest to a ranch in the interior owned by Adolpho Block, the Russian-born head of Brazil's largest publishing house. National Symphony officials could not reach him at the number given them in Rio.

The conductor-cellist's regimen during the last few months has been so intense that he has either performed, rehearsed or traveled practically every day.

During the first half of June he directed England's Aldeburgh Festival, which he took over on the death of its founder, composer Benjamin Britten.

From there he and his wife, soprano Galina Vishnevskaya, traveled to a music festival in Granada, Spain, where he played the cello and accompanied Vishnevskaya in song recitals.

From Spain it was to Los Angeles, where he participated in five different programs in about a week. In the Hollywood Bowl, he played the Dvorak Cello Concerto under Mehta, as well as the Brahms Double Concerto with violinist Itzhak Perlman. Among the concerts he conducted was an all-Tchaikovsky program similar to the one he will do at Wolf Trap, including the 1812 Overture complete with cannon.

Rostropovich arrived in Brazil July 22 to do two concerts in Sao Paulo and four in Rio, where he later added a benefit. He went to numerous dinners in his honor, a few cocktail parties and was said to be spending his free time looking for icons, samovars and other antiques, while his wife went shopping for jewels.

Brazilian pianist Nelson Freire, a friend of Rostropovich, said that in addition to fatigue, the heat affected the conductor adversely. It is winter in Rio, but temperatures have been in the upper 80s and lower 90s for the last two weeks. Rostropovich arrived tired and, according to Brazilians press reports, with a toothache.

He further complicated his schedule by spending a day at Campos do Jordao, a kind of Brazilian Interlochen where talented young musicians are trained. He lectured, listened to them play and gave an impromptu recital. The camp can be reached only by an hour helicopter ride.

A member of the Orquestra Sinfonica Brasileira said that Rostropovich himself attributed his fatique to the long flight from Los Angeles. The perfomer said "he was really getting tired by the end of the week" and that "he was getting a little short-tempered at the last couple of rehearsals," an action that is out of character with the normally ebullient musican.

The conductor signed for Friday night, Varoujan Kodjian, is the brother of NSO concertmaster Miran Kojian, though they spell their names differently. "He was here visiting Miran, and he was a good sport and agreed to take the gig." The program is Mozart, Hummel and Beethoven.

Nelson, will conduct the enormously taxing Berlioz Requiem Saturday night. Noerrr said that Nelson did the same work with his own orhcestra at the Kennedy Ceenter this year, to good notices. "Nelson knows the work cold, but it's going to be difficult for him, because he's in Aspen and we'll have to fly him here and rehearse hastily."

Rehearsals are proceeding as planned for the Friday night concert. One rehearsal will be lost for the Berlioz Requiem.

Rostropovich told officials he will arrive on Monday. If he does it will be an unusually full week, including two concerts, a labor relations hearing on his effort to dismiss 1st flutist Wallace Mann and auditions, including key spots in the cello and trombone sections.

The National Symphony series at Wolf Trap is scheduled to continue until August 25, when there will be a large-scale gala honoring the 60th birthday of Leonard Bernstein. It is the first time in recent years that the NSO's music director has taken part in its summer concerts.

The Wolf Trap management is aware that some ticket holders this weekend are going to be dissapointed by news of Rostropovich's absence. "But we are going to try to stay away for refunds," said Abie Strassler, the assistant box office manager. "We'll do it only if someone comes out and insists"