The response of LH and FSH levels to intramuscularly administered synthetic LH-RH was studied in two healthy volunteers and three oligospermic patients. Four tests with 50, 100, 250, and 500 mug of LH-RH, respectively, were carried out on each subject at 8 am; the interval between tests was one week. The serum levels of LH and FSH were determined by radioimmunoassay (double-antibody method) before each injection, and 60, 120, 180, and 240 minutes after each injection. No differences in the basal values of either hormone were observed. In both oligospermic and normal men, maximal responses were obtained with doses between 100 and 250 mug. With 500 mug, levels decreased rather than increased. Maximal peaks occurred between 60 and 180 minutes after injection. In the two normal subjects, the responses of LH and FSH were similar. Two of the three oligospermic patients showed discordant responses. From the results, we can assume that LH-RH doses between 100 and 250 mug should be used as a basis for chronic treatment.