The effects of a single injection of drugs on serum prolactin and TSH were determined in male and in estrogen-primed ovariectomized rats. The precursor of serotonin, 5-hydroxytryptophan(5-HTP), produced a significant rise in serum prolactin and TSH, whereas para chloroamphetamine, a depletor of serotonin, elicited a fall in serum prolactin and TSH. alpha-methylmetatyrosine (alpha-MMT) and reserpine, both depressors of brain catecholamine (CA) and serotonin levels, evoked significant increases in serum prolactin and reductions in serum TSH. Injection of alpha-MMT or reserpine together with 5-HTP further elevated serum prolactin but prevented any significant change in serum TSH. This suggests that the ability of alpha-MMT and reserpine to inhibit TSH release is mediated through a reduction in brain serotonin and not via a decrease in CA. alpha methylparatyrosine, which inhibits CA synthesis without altering serotonin, evoked a marked increase in serum prolactin but had no effect on serum TSH. L-dopa administration significantly reduced serum prolactin values but had no significant effect on serum TSH. Synthetic TRH increased both serum prolactin and TSH levels in male rats, but when it was injected after L-dopa administration, it did not elicit any rise in serum prolactin and evoked the same increase in serum TSH. This suggest that L-dopa acts directly or indirectly on the pituitary prolactin cells to inhibit TRH stimulation of prolactin release, but does not influence the action of TRH on pituitary TSH cells. Pilocarpine decreased serum prolactin, but had no significant effect on serum TSH values. The present study suggests that only serotonergic drugs produce similar effects on release of prolactin and TSH, whereas drugs that alter CA and pilocarpine depress prolactin release but have little or no effect of TSH.