Mstislav Rostropovich will celebrate his 50th birthday on Sunday, March 27, by conducting the National Symphony Orchestra in New York City. It will be his New York debut as a conductor.

The following night he will return to Washington as the world's foremost cellist to play a benefit concert for the Monitor Coalition. The Coalition is a group of 23 environmental and humane organizations that have joined together "to encourage cooperative action and provide a unified voice in saving marine mammals and other abused or endangered species."

For his program on Monday night, Rostropovich will play the Brahms E minor Sonata, the solo suite in C Major by Bach, the Rachmaninov Vocalise and the prokofiev Sonata in C. His planist will be Samuel Sanders, whose last Washington appearance was on March 10 at the White House. There are still tickets available for the Rostropovich recital.

There are no tickets available for Rostropovich and the National Symphony for evenings this week, but there are some still available for the Friday matinee. The program, for evenings and matinee will be the same one that Rostropovich will conduct in New York; Brahms-Haydn Variations; Haydn Cello Concerto; Prokofiev Symphony No. 3.

At the Boston Symphony concert on Friday night, the soloists will be the members of TASHI: Peter Serkin, Ida Kafavin, Fred Sherry and Richard Stoltzman. They will play in "Quatrain" by the Japanese composer Takemitsu, in the version for orchestra and quarlet. On Sunday night in their concert at the Jewish Community Center in Rockville they will play "Quartrain" in its quartet version.

You should have heard the skillful, sensitive performance of "La Boheme" led by Fred Scott - class of 1974, Georgetown University - on Saturday night with the Opera Company of Boston. It was his official debut in the regular house in Boston. He is remembered, however, for having stepped in on four hours' notice to conduct Bellini's "I Capuleti e I Montecchi" two years ago when Sarah Caldwell was unexpectedly sent off to the hospital.

Recent winners: Metropolitan Opera Auditions Finals two days ago in the Met; first prize of $5,000 to tenor Vinson Cole, 26, of New York City; second prize of $4,000 to soprano Christine DOuahue of Kingston, Pa.; third prize of $3,000 to soprano Elizabeth Parcells of Grosse Point, Mich. None of the singers was given a contract with the company.

Kathy Brake, 16, of Potomac, Md., recently won the $1,000 prize given by the Beethoven Society for pianists between the ages of 12 and 18. She is a pupil of Julian Martin.

Joseph Silverstein, concertmaster of the Boston Symphony, will appear as soloist with the Fairfax Symphony on Saturday night in Hayfield High School, playing the Tchaikovsky Concerto. He will also take part in four events on Friday at 10 a.m., noon, 2 p.m. and 4 p.m., all in conjunction with the Fairfax orchestra's concert. For details on all of these, call 941-3200.

Prince George's Civic Opera is gearing up for this weekend's "Fledermas" performances on Saturday and Sunday. The famous comedy role of the Frosch the Jailer will be handled by WTOP's intrepid, versatile, fearless, doughty Henry Tenenbaum in yet another manifestation of his many-sided genius. For details, call 277-2200, ext. 342.