To evaluate the effect of PRL on the male pituitary-gonadal system, serum concentrations of PRL, testosterone, LH and FSH were determined in healthy young men daily before, during, and after 3-day oral administration of bromocriptine, metoclopramide or sulpiride. Bromocriptine (2.5 mg as a single dose) caused, concurrently with a marked suppression of serum PRL, a significant increase of serum testosterone and a transient decrease of serum LH. The changes of PRL and testosterone were negatively correlated. With metoclopramide (10 mg q.i.d.) serum PRL was increased and testosterone inversely decreased. There was no change in LH and FSH. Sulpiride (50 mg q.i.d.) evoked the elevation of serum PRL and LH, but no change in testosterone. A significant increase in serum concentration of testosterone was also observed in a patient with PRL-producing pituitary tumour and four out of seven patients with acromegaly during bromocriptine treatment. These results suggest an inhibitory effect of PRL on testosterone secretion at the gonadal level, or direct dopaminergic stimulatory control of testosterone secretion.