SUMMARY A prolonged study relating plasma testosterone levels to coitus is reported in one male subject. Testosterone levels in samples taken during and immediately after sexual intercourse were significantly higher than those found under resting conditions. A significant difference was not apparent between testosterone measurements taken before and after orgasm, and coitus did not appear to affect plasma luteinizing hormone levels. A marked nyctohemeral rhythm was observed in testosterone levels which tended to be higher early in the day. Masturbation had no significant effect on plasma testosterone levels in seven subjects.