Prolactin response to electroconvulsive therapy (E.C.T.)was investigated in nine patients with endogenous depression. Four patients undergoing minor gynaecological surgery who had had the same type of intravenous anaesthesia as the E.C.T. group were studied as controls. 15 minutes after E.C.T. there was a 10 to 50 fold increase in serum-prolactin in eight of the patients. The effects of anaesthesia and surgery on serum-prolactin in the controls were negligible.