Synthetic luteinizing hormone-releasing hormone (LH-RH) was given in doses of 11-100 μg to normal men. Doses of 50 and 100 μg caused a two- to ten-fold rise in serum LH in all subjects, but in only 40% was the serum follicle-stimulating hormone (FSH) elevated. A significantly higher LH response to LH-RH was seen in subjects in whom a rise in serum FSH could be detected. Although there was a wide variation in the serum gonadotrophic hormone response, the test was reproducible in individual subjects. A significant rise of serum LH was seen following the injection of 1 μg LH-RH and a linear log-dose response occurred over the range 0·25 to 4 μg. There was no rise in serum FSH with the lower doses of LH-RH. Two injections of LH-RH 90 min apart produced comparable rises of serum LH and FSH. The rise of serum LH following a second injection of LH-RH was blunted when the two injections were given 30 or 45 min apart.