Hypothalamic α-melanocyte-stimulating hormone (α-MSH) was measured by radioimmunoassay in males after orchidectomy and after orchidectomy plus testosterone replacement, and in females after ovariectomy and after ovariectomy plus estradiol or progesterone, or estradiol and progesterone (EP) replacement. Gonadectomy inverted the diurnal rhythm of the α-MSH content observed in intact males in the medial basal hypothalamus (MBH) and preoptic hypothalamic area (POA), and produced a notable decrease of α-MSH total content in the three regions studied (MBH, POA and dorsolateral hypothalamus, DLH). The addition of testosterone restored the rhythm of the intact males and increased α-MSH content in MBH and POA. No diurnal variations in α-MSH content were observed in ovariectomized females. A circadian rhythm similar to that of proestrus was observed in MBH after estradiol or EP replacement, and in POA after the addition of any steroid. In DLH the injection of estradiol produced variations through the day, but they are somehow different from those described for proestrus. Treatment with progesterone significantly decreased α-MSH content in MBH and DLH, but not in POA. In this region an increase in α-MSH content was noticed after EP replacement. We conclude that gonadal steroids can alter the content of hypothalamic α-MSH and influence the diurnal variations of the peptide. This may be important in the modulation of several types of behavior or in the neuroendocrine control of gonado-tropin release in females, where α-MSH seems to modulate the release of luteinizing hormone and