The effect of progesterone on pulsatile and basal release of gonadotrophins was studied in 10 hypogonadal women. The day before progesterone treatment, control blood samples were obtained at 15 min intervals between 15.00 and 20.00 h. The next day, 50 mg of progesterone was administered im at 06.00 h and blood samples again were obtained at 15 min intervals between 15.00 and 20.00 h. Serum progesterone levels were 28.4 +/- 4.4 ng/ml at 15.00 h, 24.2 +/- 3.9 ng/ml at 17.30 h and 20.7 +/- 2.5 ng/ml (mean +/- SD) at 20.00 h. Progesterone prolonged the interval between LH pulses from 85 +/- 27 to 155 +/- 55 min (mean +/- SD), augmented their amplitude 2-fold, and caused average declines in LH and FSH concentrations of 27 and 15%, respectively. The results suggest that progesterone suppresses the mean levels of LH and FSH concentrations by acting in the brain to decrease the pulse frequency of gonadotrophin-releasing hormone.