THE NATIONAL Theatre will probably close for renovations and repairs after "Pireates of Penzance" closes in May--if a dispute over who will pay for the repairs is resolved. The theater, which was rebuilt in 1922, had two-inch cracks in the foundation, jammed fire exit doors, leaky pipes, flooding in the orchestra pit, exploding steam pipes, a leaky roo and falling chunks of plaster.

"We desperately want to keep the theater open," said board chairman Maurice Tobin. "If we can guarantee the safety of the audience, we will. But when you have a fire curtain that can't come all the way down to make the seal between the audience and the stage, and heat pipes exploding and steam coming out, you have to think of safety first."

Many of the problems have been exacerabated by the construction going on around the theater, where a $120 million hotel and office complex is being built by the Quadrangle Development Corp. and the Marriott Corporation. One memorable example is the day cement leaked into the orchestra pit from the construction site and fastened the harp to the floor during the run of "Evita."

Repairs have been held up by a continuing dispute over who should pay for them. The landlords are the Quadrangle Marriott Development Corp., who agreed to modernize the theater as part of the package they call National Place, and the Pennsylvania Avenue Development Corp., which awarded them the development contract. It is still unclear who will pay the bill, and for what. The theater, a nonprofit corporation, has a 99-year lease on the building, which is the oldest continually operating house in the country. They would expect to be reminbursed for lost revenues if the theater was closed for repairs.

"The National Theatre represents one of the real jewels in the entire Pennsylvania Avenue area and its preservation should be a priority every bit as great as the construction of new buildings," wrote Rep. Joseph McDade (R-Pa.) in a letter to PADC chairman Max N. Berry last week. McDade, a member of the approproations subcommittee, said that PADC was responsible for the repairs and has asked for an explanation of why they aren't being done.

PADC executive director Tom Regan said the repairs are clearly the responsibility of Quadrangle-Marriott and that the PADC has been meeting with them and the theater in an effort to mediate the dispute. He said the theater will not necessarily have to close, although the repairs could be made more quickly and cheaply if it did. "We do not agree that it is a threat to public safety," he said. "The theater is responsible for maintenance. If he (Tobin) has created a dangerous situation over there, we can't address that."

The needed repairs are enumerated in the lease, he said, and would cost between $2 million and $3 million.

Tobin said $475,000 was spent on paint and new seats four years ago, but none on the major structural problems. The plumbing dates from 1922 when the building was rebuilt after a fire, he said.

"We don't want to frighten people away from buying tickets for 'Pirates,' " he added. No one has actually been injured, however, just inconvenienced by puddles of water, stopped-up toilets and the like.

The fire curtain has been cited by the District fire marshal as a hazard, but is evidently not worrisome enough to stop operations. The fabric stretched across the back wall of the top balcony is to catch falling plaster. The building has settled, leaving two-inch cracks in the plaster in some areas, putting pressure on steam-carrying pipes in others.

Charles Ludlam, who runs the Ridiculous Theatrical Co. in New York, will teach a master class in "eccentric comedy" March 15 and 16 at the Source Theater. Call 462-7782 to register. It costs $50 . . . The Studio Theater has gotten a $20,000 grant from the Morris and Gwendolyn Cafritz Foundation for general operating expenses . . .

New Playwrights' is having auditions tomorrow and Wednesday for the National One-Act Play Festival. Call 232-4527."Side by Side By Sondheim" is being extended through March 20 at the d.c. space . . . Sally Crowell, who runs the Capitol Hill Arts Workshop, has come up with a new form of dinner theater as a fund-raising gimmick for the opening night of "Bus Stop" March 20. The cast will cook the food, and since the play is set in a diner they are going to serve "all-American specialties." Call 547-6839

Joseph Wiseman is Prospero in "The Tempest" at the Folger Theater opening tonight. He was in "Zalman or the Madness of God," at Arena, which is just one role in a career that started in 1938 in "Abe Lincoln in Illinois," and includes the the title role in Dr. No . . . And the Folger will host the annual secondary school Shakespeare Festival next week. If you are interested in watching high school students perform Shakespeare, there are some seats reserved for the public.

"Sugar Babies," which had its 1,000th performance on Broadway Feb. 27, is celebrating tonight with a big party. The road company of the show, a recreation of the best of burlesque, is currently playing in Baltimore.