In 14 men and 14 women we examined the relationship between melatonin and the secretion of pituitary hormones. Blood pressure, and the serum concentrations of melatonin, catecholamines, prolactin, thyrotropin, growth hormone, and cortisol were determined every 3-4 hr for 24 hr. In the normal, basal (unstimulated) condition there were no significant correlations (p greater than 0.05) between the systolic blood pressure and dopamine (r = 0.09), norepinephrine (r = 0.26), or epinephrine (r = 0.27), nor were there significant correlations between melatonin and dopamine (r = -0.01), norepinephrine (r = -0.26), or growth hormone (r = 0.17). The concentrations of melatonin correlated positively with those of prolactin (r = 0.56, p less than 0.05 for men and r = 0.58, p less than 0.001 for women) and thyrotropin (r = 0.62, p less than 0.001 for all subjects), but not with those of cortisol (r = 0.004, p greater than 0.05). We speculate that the increase in melatonin at night leads to a decrease in dopaminergic activity; the diminished release of dopamine may lead to a simultaneous increase in thyrotropin and prolactin.