In a previous report, a consistent hypoprolactinemic effect of p-tyramine was observed in male rats under several experimental conditions in vivo. In the present experiments the action of p-tyramine on PRL release in vitro, or after challenge with different hyperprolactinemic drugs (serotonin, morphine, and TRH) was tested. Furthermore the participation of octopamine, a metabolite of tyramine, was evaluated with regard to the hypoprolactinemic action of the amine. P-Tyramine inhibited PRL release from hemipituitaries incubated in vitro at doses of 10(-4) and 10(-6) M (inhibition to 31% and 59% of control values, respectively). When tested for its ability to displace [3H]spiperone binding in vitro to a crude fraction of anterior pituitary membranes it was found that it did not compete with the D2 receptor labeled by [3H]spiperone, even at the concentration of 10(-4) M. P-Tyramine (40 mg/kg) antagonized the elevation of serum PRL levels by morphine, serotonin, and TRH. On the other hand, octopamine, which is formed from tyramine, also inhibited high PRL values found after stress, though the effective dose was higher than that of tyramine. Pretreatment with diethyldithiocarbanic acid, which inhibits conversion of p-tyramine to octopamine, did not modify the effect of tyramine in stress. The present results indicate that tyramine can inhibit PRL release due to certain drugs, by acting directly at the pituitary level. It does not displace [3H]spiperone binding from anterior pituitary membranes, and octopamine which lowers PRL release itself, cannot account for the effect of tyramine.