Urinary excretion of gonadotropins increases during fasting. We investigated whether this increase results from increased pituitary secretion of LH and FSH or from altered renal excretion of protein molecules. To this end, we studied urinary gonadotropin excretion, serum gonadotropin levels, and pituitary responsiveness of LHRH during control, 10-day fasting, and refeeding periods in 10 mildly obese postmenopausal women. Additionally, we measured urinary cortisol and estriol excretion and circulating levels of dehydroepiandrosterone sulfate, estradiol, estrone, melatonin, norepinephrine, epinephrine, and dopamine during the control, fasting, and refeeding periods. While urinary excretion of gonadotropins increased markedly during fasting, there were no significant changes in serum gonadotropin levels or in the pituitary sensitivity to LHRH. Plasma norepinephrine and serum melatonin increased significantly during fasting, but serum and urinary estrogens, indices of adrenal activity, and plasma levels of epinephrine and dopamine did not change. These results show that the stress of short term fasting selectively activates only certain components of the neuroendocrine system without any appreciable changes in the function of the gonadotropin-secreting system. Fasting-induced gonadotropinuria is probably explained by altered renal handling of gonadotropin molecules.