8-OH- DPAT (8-hydroxy-2-[di-n-propylamino] tetralin) is a novel aminotetralin derivative which has been proposed to be a serotonin (5-HT) agonist devoid of dopamine agonist effects. We now report that the administration of 8-OH- DPAT , like known 5-HT agonists, produced a rapid elevation of serum prolactin concentrations in male rats. The prolactin response to 8-OH- DPAT , like that induced by other 5-HT agonists, was greatly potentiated in animals pretreated with the tryptophan hydroxylase inhibitor, para-chlorophenylalanine. However, the 8-OH- DPAT -induced elevation of serum prolactin concentrations in untreated rats was not dose-dependent and was modest in magnitude compared to that produced by known 5-HT agonists. In contrast to the stimulatory effects of 8-OH- DPAT on prolactin secretion in vivo 8-OH- DPAT suppressed the secretion of prolactin from anterior pituitary tissue in vitro, and this effect was blocked by haloperidol. The results of the present study are supportive of the view that 8-OH- DPAT has dopamine agonist, as well as 5-HT agonist, properties.