SCHWELLNUS, M. P. and N. F. GORDON. Effect of opioid antagonism on esophageal temperature during exercise. Med. Sci. Sports Exerc, Vol. 20, No. 4, pp. 381-384, 1988. The effect of naloxone-induced endogenous opioid antagonism on core temperature control during exercise was studied in 8 competitive cyclists. Volume-matched infusions of placebo or 2 mg of naloxone were administered, in a randomized double-blind crossover fashion, prior to maximal graded cycle ergometer exercise testing. Esophageal (Tes), rectal (Tre) and oral (Tor) temperatures were measured before and after exercise, and Tes was continuously measured during exercise. Cardiorespiratory responses and maximal exercise duration were unaffected by naloxone. Naloxone did not significantly modify the Tes response or the highest Tre and Tor elicited by exercise testing. The rise in Tes (placebo: 2.2 ± 0.4°C; naloxone: 2.1 ± 0.4°C) and Tre (placebo: 1.1 ± 0.2°C; naloxone: 0.9 ± 0.4°C) was significant (p<0.001) with and without naloxone, whereas the rise in Tor was significant (p<0.05) with placebo (0.5 ± 0.5°C) but not with naloxone (0 ± 0.8°C, p>0.5). These data indicate that although 2 mg of naloxone might alter heat exchange in the oral cavity during exercise, it does not modify the actual core temperature response. Therefore, insofar as this dosage of naloxone may be used to examine the functional role of opioid-mediated mechanisms, our findings are not tenable with the hypothesis that endogenous opioids play a role in maintenance of thermal homeostasis during exercise.