To try to determine the site and mode of action of dopamine (DA) on the secretion of gonadotropins in humans, 4 series of studies were performed in 19 healthy volunteers (7 healthy males, 5 females in follicular phase and 7 postmenopausal females). The intravenous infusion of DA at the rate of 4 micrograms/kg/min decreased plasma LH levels to 75.9 +/- 3.7 (mean +/- SE)% of the initial levels. The oral administration of 500 mg of levodopa (Dopa) also suppressed the concentration of plasma LH to 73.0 +/- 3.5%, but the pretreatment with two doses of 100 mg each of carbidopa (CD), a peripheral dopa decarboxylase inhibitor, attenuated this suppressing effect of Dopa on LH levels (nadir 82.4 +/- 4.1). Plasma FSH were not significantly altered by these drugs. On the other hand, the concentration of endogenous LRH in peripheral blood remained unchanged throughout the studies. The administration of CD alone had no effect on any of these 3 parameters. These results would suggest the direct suppressing effect of DA on pituitary gonadotrophs, although the modulation at the level of the hypothalamus also cannot be ruled out.