In a Kensington warehouse close to firms specializing in draperies, chemicals and building equipment, conductor Peter Ernst Lassen raised his baton yesterday before the 45 members of the Royal Danish Ballet Orchestra.

The next sound in the dusty warehouse neighborhood was that of professional musicians breezing through "Napoli," written by a 19th century Danish composer. They were rehearsing for their three-night engagement at Wolf Trap, which begins tonight.

Since Rehearsal Spaces Inc. opened at 10417 Metropolitan Ave. five years ago, Kensington has frequently hosted artists headed for the Washington area's grander stages.

There are "an average of two rehearsals a day, seven days a week," says Michael Toperzer, manager of the rehearsal facility. "We get busy in spring and summer. In winter we do mostly 'Nutcrackers,' " including those put on by the American Ballet Theatre and the Washington Ballet.

In addition to rehearsal space, the company rents percussion equipment, music stands and electronic sound "mixers."

This summer it is supplying equipment for the Folklife Festival on the Mall, the Ballet National de Marseille at the Kennedy Center, the Carter Barron Amphitheater and Merriweather Post Pavilion, in addition to Wolf Trap.

Beverly Sills, Aretha Franklin, the Joffrey Ballet, the New York City Opera, new-wavers like Four Out Of Five Doctors and rhythm and blues bands like Parlliment/Funkadelic all have hit the high notes at Rehearsal Spaces.

The company grew out of Toperzer's father's retail business, Drums Unlimited, which he started 25 years ago on St. Elmo Avenue in Bethesda. Frank Toperzer still runs it and plays drums professionally, while Michael quit drumming to snare more business through Rehearsal Space's rental operation.

Michael Toperzer says business has quadrupled in five years.

The two studios in the warehouse have raised parquet floors for dancers and eight-inch-thick walls for musicians. The walls of one studio are angled for better acoustics and hung with special sound baffles.

"Once we had Louis Bellson's big band in one studio and a 40-piece orchestra in the other and they never knew each other were there until they met in the lounge during a break," Michael Toperzer said.

Yesterday the lounge was occupied by the card-playing 7-year-old twin daughters of second violinist Martha Mott.

The $15 per hour Toperzer charges for rehearsal space "just about covers the rent and utilities," he said. Equipment rental is the moneymaking part of the business, he said..

Equipment ranges from a wind machine--"I had to make two of those for a Nureyev ballet at the Kennedy Center,"--to four celestas, keyboard instruments with steel bars struck by piano hammers that Toperzer said are used mainly in performances of "The Dance of the Sugar Plum Fairies" in the "Nutcracker" to assorted gongs, chimes, castanets, whistles, triangles and things called vibra-slaps.

Yesterday, Toperzer was on the roof fixing the air conditioning, in the studio lowering conductor Lassen's music stand and in the office answering questions from a caller who wished to rent a marimba for four months.

The orchestra played from 1 p.m. until 4:30, when Rehearsal Spaces Inc. fell silent for an hour and a half. A rhythm and blues band moved in at 6.