Through an innovative project of Washington's National Symphony Orchestra (NSO), 70 young area musicians, including 30 from Prince George's and Montgomery counties, now know first-hand what it means to be a member of a major professional orchestra.

Early this month, the NSO sponsored its second annual Youth Orchestra Day at the Kennedy Center Concert Hall. Players from seven youth orchestras were given the opportunity to perform with 50 members of the NSO under the direction of Christian Badea, the symphony's Exxon-Arts Endowment conductor. During the 2 1/2 hour session the young players and their professional partners read through works by Brahms, Schumann and Tchaikovsky.

Participating orchestras include Prince George's Youth Orchestra, Montgomery County Youth Symphony, Masterworks Orchestra of Montgomery County, D.C. Youth Orchestra, Northern Virginia Youth Orchestra, Annapolis Senior High School Orchestra and Saverna Park High School Orchestra, Ann Arundel. The conductor of each orchestra selected ten players to participate in the Kennedy Center rehearsal.

Trombonist Brad Martin, a senior at Rockville High School, was one of those from Masterworks Orchestra. Like many of his fellow participants Brad began his music career in elementary school.

""I wanted to play trumpet," said Brad, "but the teacher told me I had fat lips. He said I would be ideal for the trombone."

Brad, as most of the other young musicians, has had extensive performing experience. He has been a member of the Maryland All-State Orchestra and the Maryland All-State Band. He also has played with professional musicians, including a recent informal session with the Air Force Jazzmen at Rockville High School.

After hearing Brad, the Jazmen's director encouraged him to play trombone for the Air Force, Brad now plans to audition for the Airmen of Note which, he says, is the Air Force's top band.

When asked about the NSO performance, he replied with a air of calm professionalism: "I'm looking forward to sitting next to John marcellus," (Marcellus is the NSO's principal trombone player.)

This serious, informed approach is typical of the young musicians. They all must be able to juggle many hours of practice and performance with acadamic schedules.

Violinist Paul Cumberland of the Prince Georg's Youth Orchestra is a senior at Friendly High School. He practices 3 1/2 to 4 hours a day, goes to bed as late as midnight and sometimes gets up at 5 a.m. to study.

Although their music schedules are demanding, some of the young performers manage other activities. For instance, cellist Chris Ishee of the Prince George's Youth Orchestra is on the cross-country team at Parkdale High School, and violinist Mary Ellen Spencer of the Montgomery County Youth Symphony, spends her afternoon practicing for cross country and track teams at Seneca Valley High School.

Before the rehearsal began many of the young musicians conceded to some anxiety. Asked if he were nervous, Douglas Dube, a violinist from the Masterworks Orchestra, replied: "Yes, very. To play with a professional orchestra - I wouldn't have dreamed of doing that two years ago."

David Thomas, a clarinetist from the Masterworks Orchestra said: "I just hope I can do the the music justice. I look forward to meeting the players. I'm always interested in a professional's opinion."

But when conductor Badea arrived and the rehearsal began, the nervousness was forgotten. Whatever the performance may have lacked in subtlety and accuracy - many of the young players were seeing the music for the first time - was offset by the musicians' enthusiasm.

Afterwards, the excitement still very much with them, the young players tried to convey with the morning had meant.

"It's a different thing to follow the professional conductor," said Douglas Dube. "The players are very good. . . It's a big sound, a very big sound."

"My stand partner was really nice about my mistakes," said Mary Ellen Spencer, smiling.

"It was a lot of notes to sightread," said Chris Ishee, "but it was really good . . fsstimulating. I played the best I could." Better than he thought he could? He paused a moment and then said softly: "Yes, I played over my head."

And Paul Cumberland found one word to say it all: "Wow!"

Maryland students participating in the rehearsal:

Montgomery County Youth Orchestra: Mary Ellen Spencer, Alice Hollister, violing; Seth Low, Laura Thielke, cello; Dawn McGee, viola; Paul Miedema; string bass; Sarah Phillip, flute; Susan Rouse, clarinet; Ed Cervenka, trumpet; paul Coheh, trombone

Prince George's Youth Orchestra: Kathy Auer, Paul Cumberland, violin; David Miller, Theresa Renner, viola; Lynn Angebrandt, Christopher Ishee, cello; Geoffrey Harper, doublebass; Diane Dextradeur, flute; Carol Folea, clarinet; Ronald Cancelose, tuba.

Masterworks Orchestra: Douglas Dube, Anna Kuwabara, Julie Forrest, Leon Turkevich, violin; Leigh Pilzer, Andrew Pollner, cello; Kathie Stewart, flute; David Thomas, clarinet; Don Shore, Jr., bassoon; Brad Martin, trombone.