M-Chlorophenylpiperazine (m-CPP) produces effects on the central serotonergic system in animals compatible with direct agonist activity on postsynaptic serotonin receptors. Although it is a metabolite of the antidepressant trazodone, m-CPP has not previously been given to humans. To evaluate the neuroendocrine, behavioral, and physiological effects of m-CPP, 15 normal subjects were given 0.5 mg/kg m-CPP, orally. Administered acutely under double blind, placebo-controlled conditions, m-CPP was well tolerated by 14 of the 15 subjects; it produced significant increases in plasma PRL and cortisol and in body temperature, without changing pulse or blood pressure. The mean (SD) maximal increases over baseline for PRL, cortisol and temperature were 13.4 (9.9) ng/ml, 10.1 (6.7) micrograms/100 ml, and 0.4 (0.2) C, respectively. A small but significant increase in self-rated activation-euphoria and anxiety was noted by some subjects, whereas there were no significant effects on ratings of depression, dysphoria, altered self-reality, or functional impairment. These results are similar to those for other serotonin agonists and, thus, suggest that m-CPP merits further study as a pharmacological probe of serotonergic responsivity in humans. The results also support the hypothesis that serotonin plays a role in the regulation of PRL, cortisol, body temperature, and mood.