The effects of daily evening melatonin injections on serum and pituitary levels of growth hormone (GH) and follicle stimulating hormone (FSH) were investigated in male Syrian hamsters receiving thiourea in the drinking water. Melatonin injections, by themselves, had no significant effect on serum or pituitary GH. Thiourea induced hypothyroidism reduced pituitary GH content but increased serum GH several fold. Daily thyroxin (T4) injections for 3 weeks partially restored pituitary GH content and reduced circulating GH to control values. Melatonin injections prevented T4 from reducing circulating GH levels to normal in hamsters receiving thiourea. As previously reported, FSH levels in serum and pituitary were reduced by melatonin. Thiourea-induced hypothyroidism prevented this effect. Daily T4 injections increased circulating FSH levels above control levels; melatonin injections prevented this increase in serum FSH. These observations show that melatonin and T4 have antagonistic actions on GH and FSH release from the pituitary. We conclude that melatonin influences the release of hypothalamic hormones regulating GH and FSH release from the pituitary. The effects of T4 on the sensitivity to melatonin injections could be accounted for by thyroid hormone regulation of pituitary receptors for hypothalamic hormones. An alternative explanation is that T4 regulates the concentration of melatonin receptors in the central nervous system.