The purpose of this study was to determine whether plasma oxytocin (OT) levels change during human sexual responses and, if so, to demonstrate the temporal pattern of change. Plasma OT levels were measured by RIA before, during, and after private self-stimulation to orgasm in normal men (n = 9) and women (n = 13). Blood samples were collected continuously through indwelling venous catheters. The subjects pressed a signal to indicate the start and finish of orgasm/ejaculation. Objective assessment of sexual arousal and orgasm was obtained by measuring blood-pulse amplitude and electromyographic activity, recorded continuously throughout testing from an anal device containing a photoplethysmograph and electromyograph electrodes connected to a polygraph located in an adjacent room. These measures allowed collection of data from men and women of changes in blood flow and muscle activity in the lower pelvic/pubic area. Plasma OT levels increased during sexual arousal in both women and men and were significantly higher during orgasm/ejaculation than during prior baseline testing. We suggest that the temporal pattern of secretion could be related to smooth muscle contractions of the reproductive system during orgasm.