The effects of analgesic doses of morphine on ventilation, arterial blood gas tensions, chemical control of breathing, and the ventilatory response to exercise were studied in six normal subjects. After administration of 0.2 mg/kg morphine, resting ventilation decreased primarily because of a reduction of tidal volume. Ventilatory responses to carbon dioxide and hypoxia were significantly reduced to one-half and one-third of control, respectively. Ventilatory responses at any given level of exercise were significantly reduced after morphine. However, since oxygen consumption during exercise was similarly reduced after morphine, the relationship between ventilation and metabolic rate during steady-state exercise was not altered by the drug. In addition, morphine prolonged the attainment of steady-state ventilation in four of the six subjects, similar to that reported for chemodenervated subjects. The findings suggest that blunting of chemoreception for hypoxia and hypercapnia has no effect upon the link between metabolic rate and ventilation during steady-state exercise, but the hypoxia chemoreflex may be involved in determining the dynamic characteristics of the response.