The agonist analogue of gonadotropin-releasing hormone (GnRH), [(imBz1)-D-His6,Pro9-NEt]GnRH, has a potency 200 times that of the native hormone in vitro. In single dose studies in man, this analogue resulted in 2- to 4-fold elevation of LH and FSH, and demonstrated a prolonged duration of activity. [(imBz1)-D-His6,Pro9-NEt]GnRH appears to be safe and, as with other analogues of GnRH, may have application to clinical medicine.