The hormonal responses to single subcutaneous injection of the GnRH agonist nafarelin have been shown to have considerable potential as a diagnostic test in a number of settings. Since nafarelin injection is no longer produced, studies were conducted to determine the dosage of leuprolide that would yield equivalent responses. Nafarelin 100 micrograms stimulates LH and FSH for 24 h, releasing about 7-fold more gonadotropin than this dose of natural GnRH, which accounts for its ability to elicit gonadal steroid responses. Normal adult men and women were randomized to receive leuprolide doses of 0.1, 1.0, or 10 micrograms/kg; a study extension evaluated doses up to 20 micrograms/kg in men. The responses of LH, FSH, testosterone, and estradiol were monitored for 24 h, and the data were compared to those previously obtained on nafarelin. Leuprolide dose of 10 micrograms/kg yielded LH responses similar to 1-1.5 micrograms/kg nafarelin. However, this leuprolide dose unexpectedly released less FSH than nafarelin. Nevertheless, the gonadotropin responses were sufficient to elicit equivalent or greater sex steroid responses to leuprolide. These studies also further delineated sex-specific differences in pituitary responsiveness to challenge with GnRH agonists: men had a significantly lower baseline FSH level, greater LH release within the first hour, and lesser secretion of LH and FSH over the 24-h period. These studies indicate that leuprolide in a dosage of 10 micrograms/kg would be expected to be efficacious in testing the pituitary-gonadal axis in men and women.