To determine the role of opioids in modulating the ventilatory response to moderate or severe hypoxia, we studied ventilation in six chronically instrumented awake adult dogs during hypoxia before and after naloxone administration. Parenteral naloxone (200 micrograms/kg) significantly increased instantaneous minute ventilation (VT/TT) during severe hypoxia, (inspired O2 fraction = 0.07, arterial PO2 = 28-35 Torr); however, consistent effects during moderate hypoxia (inspired O2 fraction = 0.12, arterial PO2 = 40-47 Torr) could not be demonstrated. Parenteral naloxone increased O2 consumption (VO2) in severe hypoxia as well. Despite significant increases in ventilation post-naloxone during severe hypoxia, arterial blood gas tensions remained the same. Control studies revealed that neither saline nor naloxone produced a respiratory effect during normoxia; also the preservative vehicle of naloxone induced no change in ventilation during severe hypoxia. These data suggest that, in adult dogs, endorphins are released and act to restrain ventilation during severe hypoxia; the relationship between endorphin release and moderate hypoxia is less consistent. The observed increase in ventilation post-naloxone during severe hypoxia is accompanied by an increase in metabolic rate, explaining the isocapnic response.