Considering the importance of the 21-OH group in promoting the anti-inflammatory activity of the steroids, it is possible that differences in the rate of hydrolysis of steroid esters may influence the potency of pharmacological activity of the steroids (Polley and Mason, 1950, Goldfien, et al., 1955). In the present paper the rate of de-esterification of the steroid esters in human synovial fluids was studied in vitro. Hydrolytic rates of dexamethasone-21-sulfate (DS) and dexamethasone-21-phosphate (DP) in synovial fluids aspirated from the knee joint of 10 patients each with rheumatoid arthritis and osteoarthritis of the knee measured by radioimmunoassay. DP was hydrolyzed to the extent of 10%, 30%, 50%, 80% after incubation for 1, 3, 6, 24 hours respectively. On the other hand, DS was hydrolyzed to the extent of less than 10% even after incubation for 24 hours. The difference between the rate of de-esterification of DP and DS for each period was statistically highly significant (p less than 0.001). These results suggest that DP is hydrolyzed in synovial fluids much faster than DS and converted into free dexamethasone which may exhibit a powerful anti-inflammatory action.