This study assessed the role of endogenous opiate systems in the hyperglycemic response to endotoxin challenge in mice. Blockade of opiate receptors by administration of the opiate antagonists naloxone (1.0 mg/kg) or naltrexone (1.0 or 5.0 mg/kg) significantly lessened to degree of hyperglycemia cause by endotoxin challenge (80 micrograms). Methyl naltrexone, a peripherally acting opiate antagonist, had no demonstrable effect on endotoxin-induced hyperglycemia. Finally, induction of tolerance to morphine prevented the hyperglycemic response to endotoxin challenge. These results suggest a causative role for central nervous system endorphinergic mechanisms in the hyperglycemic response to endotoxin administration. They support the view that centrally acting opiate antagonist, by blocking the brain opiate receptors, can influence metabolic adaptation to endotoxin shock.