Supreme Court Justice William J. Brennan Jr. returned to Washington last night after physicians at the Mayo Clinic in Rochester, Minn., concluded that he does not have prostate cancer.

Meanwhile, Justice Thurgood Marshall, 79, was admitted to the Bethesda Naval Hospital yesterday afternoon for treatment of a blood clot in his right foot, Supreme Court spokeswoman Toni House said.

"He is in good condition, according to his physicians, and is expected to be hospitalized approximately a week because he is receiving intravenous anticoagulants, which require monitoring," House said.

Brennan, 81, the court's senior liberal, said in a statement that doctors had "completed the evaluation of my prostate condition.

"Although the prostate is somewhat enlarged, the condition is not malignant and no surgery is required. In addition, the general examination was very satisfactory."

Brennan was admitted to the clinic on Sunday to undergo testing the next day. He said in a statement released Monday that he intended to participate in the court's 1987 term, which begins the first Monday in October.

Justice Harry A. Blackmun is scheduled to return next month to the clinic for further treatment of a recurrence of prostatic cancer. Blackmun, 78, finished the first half of his treatment last week.