Ethanol depresses the ventilatory responses to hypercapnia and hypoxia. We hypothesized that this ventilatory depression, like some other central nervous system effects of ethanol, might be mediated via endorphins. In a double-blind placebo-controlled study, we assessed the effect of the opiate antagonist naloxone on ventilatory responses during ethanol intoxication in 18 normal men. Standard rebreathing studies were done at baseline, after ethanol (1.5 ml/kg, p.o.), and after each of 2 intravenously administered injections. One of the injection sequences PP, NP, or PN (N = naloxone, 0.8 mg; P = placebo, 2 ml) was randomly assigned to each subject. The ventilatory responses were reduced after ethanol administration compared with those at baseline (p less than 0.05). In groups NP and PN, naloxone restored the hypercapnic response (p less than 0.05). Placebo injection did not significantly alter the response slopes. Hypoxic ventilatory responses showed the same trends but did not reach statistical significance. This study shows that naloxone reverses ethanol-induced depression of hypercapnic drive, suggesting that an opiate-mediated mechanism is responsible for this depression.