The effect of surgical stress on the secretions of LH, FSH, testosterone (T) and oestradiol (E2) were studied in twelve male patients. During surgery LH rose significantly; post-operatively, LH fell but remained persistently elevated a week after operation. However, T and E2 fell progressively to a nadir on the second and fifth post-operative day respectively and remained suppressed. Serum FSH showed no significant change. Despite a post-operative decrease in sex hormone binding globulin (SHBG) binding capacity, non-SHBG bound T showed a decrease parallel to T. Multiple sampling studies showed that the secretions of LH were increased and that of T were decreased post-operatively. Following surgery, LH responses to LHRH were magnified, FSH and T responses showed no significant change when compared with the pre-operative responses. These data suggest that secretions of LH were increased during surgery. Following surgical stress, T and E2 concentrations were suppressed resulting in a compensatory elevation of LH concentrations.