White House press secretary James S. Brady underwent urgent surgery last night to keep a worrisome series of blood clots away from his heart and lungs.

The operation, the third for Brady since he was shot March 30 as he stood next to President Reagan outside the Washington Hilton Hotel, began at 9:10 p.m. and ended shortly after midnight.

Brady was reported in satisfactory condition early this morning.

Doctors at George Washington University Hospital inserted a small, umbrella-like sieve in one of Brady's major blood vessels to block any more clots.

A pain in Brady's chest yesterday morning led his doctors to begin a daylong series of X-rays and other studies, and the cause turned out to be what Dr. Dennis O'Leary, GW's medical spekesman, called "several small blood clots" in the right lung.

The clots, or thromboembolisms apparently had formed in Brady's legs and traveled through his veins, then his heart to his lung.

Clots such as these are "an occupational risk of being in bed," O'Leary explained. Brady has been bedridden virtually all of the time since he was shot.

A week ago, concerned about a possible buildup of air inside his skull, his doctors ordered complete bed rest at a 20-degree angle to keep his head high.

"What you get," said O'Leary, "is some pooling of blood in the legs, and clots like these can form."

The fact that some clots reached Brady's lung was "in no way life-threatening or imminently serious" in itself, O'Leary said.

The danger, he said, is that a larger clot will form in a leg, travel through the right side of the heart -- where used blood is pumped to the lungs for more oxygen -- and block the pulmonary artery that should carry the blood to the lungs.

Only yesterday morning, on word from Brady's doctors, the White House declared that he had enjoyed "a good weekend" and was continuing to recover from his second brain operation, to block passage of air to the brain on April 22, and events since then.

Antibiotic therapy to prevent infection had been halted Friday, and Brady's temperature had remained normal.

Then, yesterday morning, O'Leary said, Brady developed "a vague chest discomfort," and a chest X-ray soon showed "a very small infiltrate or little density" on the edge of the lung.

Further studies, including a "lung scan" using radioactive chemicals and a pulmonary angiogram, or lung X-ray using a contrast dye, revealed several such areas.

Brady's surgeons then took what O'Leary called the "most reasonable approach." They placed the coneshaped metal sieve -- about one-half to three-fourths of an inch in diameter and 2 1/2 inches long -- in the vena cava, or large vein that carries all of the blood from both legs to the heart and lungs.

The incision for the sieve was made in Brady's right groin, and the sieve was threaded along a wire to a point just about at the navel.

The sieve consists of a cone of pins, much like bobby pins, that spread out to fill the vein. The sieve, it is hoped, will permit blood to pass while preventing passage of clots. Then any further clots and clots already in the llung should dissolve.