BACKGROUND: The purpose of this study was to define morphine's effects on resting ventilation and the ventilatory response to hypoxia and hypercarbia. METHODS: Six healthy nonsmoking young adult males were tested for the respiratory effects of intravenous morphine (0.15 mg/kg). Test began with baseline measurement of resting ventilation, isocapnic hypoxic ventilatory response (HVR), and normoxic hypercapnic ventilatory response (HCVR). After baseline measurement, morphine was administered and ventilatory responses were determined 20 and 40 min postinfusion. RESULTS: Morphine significantly decreased resting ventilation, hypoxic ventilatory response, and hypercarbic ventilatory response. Resting hypoventilation manifested as a peak rise in PETCO2 from 38.0+/-1.4 to 42.8+/-1.0 mmHg ( SEM) at 20 min (p<0.05). Hypoxic ventilatory response, measured as the slope of the ventilatory response to hypoxia, decreased from a control of 20.7+/-3.8 to 14.5+/-7.2 at 20 min after morphine (p<0.05). Hypercapnic ventilatory response, measured as the slope of the ventilatory response to hypercarbia, also decreased from 34.9+/-7.5 to 11.1+/-4.9 (p<0.05) 20 min after morphine. CONCLUSION: These decreased responsiveness to the chemical stimuli to breathing may contribute to the ventilatory depression frequently seen after administration of morphine.