The role of endogenous opioids in the control of thyroid-stimulating hormone (TSH) secretion in uremic male rats was investigated using the narcotic antagonist naloxone. In order to eliminate the effect of weight loss due to uremia-induced anorexia as a cause of altered TSH secretion in uremia, we also studied a group of normal control animals who were pair-fed with the uremic animals, so that their weights were comparable to that of the uremic animals. Naloxone administration produced a significant increase in the basal concentration of TSH in the uremic rats, but had no effect on the baseline TSH concentrations in the other groups of animals. The peak TSH response to TRH (5 µg i.v.) was significantly blunted in the uremic animals compared to the normal controls and the starved animals. Naloxone administration did not alter the peak thyrotropin-releasing hormone (TRH) stimulated TSH response in any of the experimental groups of rats. Because of the possibility that the effects of naloxone on TSH secretion in the uremic rats were related to impaired clearance of the naloxone in those animals, an additional group of normal rats was given twice the dose of naloxone administered to the uremic animals. The higher dose of naloxone was similarly without effect on the basal or TRH-stimulated TSH secretion in this group. The data suggest that experimental renal failure is associated with diminished sensitivity of the thyrotroph to TRH stimulation and that this blunted sensitivity cannot be abolished by blockade of endogenous opioids by naloxone. Opioid blockade does, however, increase basal TSH secretion in uremic animals, suggesting an increase in endogenous opioidergic tone in uremia. The increase in TSH secretion following opioidergic blockade in uremic rats may be related to an increase in endogenous TRH secretion following opioid blockade