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Although hemorrhagic shock decreases the minimum alveolar concentration (MAC) of inhaled anesthetics, it minimally alters the electroencephalographic (EEG) effect. Hemorrhagic shock also induces the release of endorphins, which are naturally occurring opioids. We tested whether the release of such opioids might explain the decrease in MAC.Using the dew claw-clamp technique in 11 swine, we determined the isoflurane MAC before hemorrhage, after removal of 30% of the estimated blood volume (21 mL/kg of blood over 30 min), after fluid resuscitation using a volume of hydroxyethylstarch equivalent to the blood withdrawn, and after IV administration of 0.1 mg/kg of the μ-opioid antagonist naloxone.Hemorrhagic shock decreased the isoflurane MAC from 2.05% ± 0.28% to 1.50% ± 0.51% (P = 0.0007). Fluid resuscitation did not reverse MAC (1.59% ± 0.53%), but additional administration of naloxone restored it to control levels (1.96% ± 0.26%). The MAC values decreased depending on the severity of the shock, but the alterations in hemodynamic variables and metabolic changes accompanying fluid resuscitation or naloxone administration did not explain the changes in MAC.Consistent with previous reports, we found that hemorrhagic shock decreases MAC. In addition, we found that naloxone administration reversed the effect on MAC, and we propose that activation of the endogenous opioid system accounts for the decrease in MAC during hemorrhagic shock. Such an activation would not be expected to materially alter the EEG, an expectation consistent with our previous finding that hemorrhagic shock minimally alters the EEG.