The mode of action of intraarticularly administered corticosteroids was studied. Fifteen patients with osteoarthritis and painful effusions had one knee injected with normal saline, triamcinolone hexacetonide 20 mg or prednisolone tebuate 20 mg. Fifteen millicuries of 99mTc-labeled albumin was given intravenously before and at various time intervals after the injection of the depot corticosteroid or saline. Whole blood and synovial fluid (SF) samples were used for blood to SF count rate ratio determinations. SF white blood cell counts (WBC) were also performed. The results indicated that intraarticular depot corticosteroids decreased synovial permeability and caused increases in the SF WBC. Moreover, decreases in pain and knee swelling correlated better with a decrease in vascular permeability, as assessed by the count ratio, than with SF leukocytosis.