Plasma concentrations of betamethasone, cortisol and corticosterone were measured before and after intraarticular injection of a betamethasone-depot preparation (Celestan-Depot) by radioimmuno-assay in 31 patients. Plasma concentration of betamethasone reached its maximum of between of 10 and 17 microgram/dl 30 min after injection. It had fallen to half after 2 hours, and practically to nil from the eighth day onwards. Lowest plasma levels of cortisol and corticosterone occurred after 6--24 hours, returning to the normal range after four days. In nine patients with knee-joint effusion and synovitis the plasma concentration of betamethasone was significantly higher after 24 hours, and cortisol and corticosterone values after 48 hours significantly more suppressed, than in patients without joint effusions and signs of inflammation. The results indicate that plasma concentration of betamethasone and the suppressant effect on the adrenal cortex after intra-articular injection is similar to that after intramuscular applications. Correspondingly, systemic application of the cortisol derivatives can cause significant side effects and be contra-indicated also after intra-articular injection.