Sen. Joseph R. Biden Jr. (D-Del.) has been hospitalized at Walter Reed Army Medical Center for treatment of a blood clot in his lung, a hospital spokesman said yesterday.

Biden, 45, who underwent emergency surgery last month to repair an aneurysm in an artery supplying blood to his brain, was rushed to the hospital by ambulance Sunday night from his home in Wilmington with what initially was reported to be a blood clot in his leg.

Tests revealed that a clot had lodged in one of his lungs, where it "did no damage," according to a hospital statement. Biden's doctors were doing further tests yesterday to decide whether he should receive anticoagulant drugs or have a filter implanted in an abdominal vein to prevent additional clots from reaching the lungs.

Experts interviewed yesterday said the choice of treatment is complicated by Biden's recent brain surgery and by the fact that his surgeons had planned to operate again next month to repair a second, smaller aneurysm. An aneurysm is a balloon-like swelling in the wall of an artery.

Anticoagulant drugs such as heparin are the preferred treatment for a blood clot in the lung, but they would increase the chance of bleeding at the site of the recent operation, according to a prominent neurosurgeon who asked not to be identified. "There's a very great dispute as to how to do this," he said.

Biden, expected to remain in the hospital for about a week, was in good spirits yesterday but "not particularly happy to be here," according to a hospital spokesman. He added that doctors have not decided whether to postpone Biden's second brain operation because of the complication.

Sources said Biden had begun to increase his activity last week and experienced pain over the weekend. Doctors familiar with his case said the period of bed rest after his operation Feb. 11 probably contributed to formation of a blood clot in his leg, a common surgical complication.

Pieces of such clots can break off and travel through the heart to lodge in the lung, a so-called pulmonary embolism. It is estimated that 500,000 Americans have pulmonary embolisms each year and that 50,000 of them die.

Heparin, commonly used as initial therapy, helps to dissolve the clot and prevent additional clots from forming, said Dr. Richard Waldhorn, an associate professor of medicine at Georgetown University Medical School. He said it is usually given intravenously for seven to 10 days. Patients then are given warfarin, an oral medication that reduces production of blood-clotting proteins by the liver, for as long as six months.

Because such medications could raise the risk of bleeding at the site of Biden's recent surgery and complicate plans for his second operation, Waldhorn said doctors might choose an alternative treatment, implanting a device inside a large abdominal vein to prevent blood clots from reaching his lungs.

He said the metal filter, which resembles a tiny umbrella without a fabric covering, is inserted through a neck vein, using local anesthesia. It is then passed through the right side of the heart and into the inferior vena cava, the large abdominal vein carrying blood from the legs. It remains in place permanently.

Biden, chairman of the Senate Judiciary Committee, suffered neck pain in late January because of bleeding from an aneurysm at the base of his brain. He and his doctors initially ascribed the pain to a pinched nerve. He had an episode of more severe neck pain and nausea Feb. 9 during a speaking engagement, and doctors at Walter Reed discovered the aneurysm.

Sources familiar with the case said that Biden suffered no neurological complications from the bleeding or the brain surgery and that he has been fortunate. One large study found that 70 percent of individuals with bleeding from such an aneurysm die or are disabled.