Resting ventilation, arterial pH and gas tensions in the arterial blood and ventilatory responses to transient O2 inhalation were studied by plethysmography, under normoxic and hypoxic (FIO2 = 0.12) conditions, in the awake rat before and after chronic bilateral denervation of the carotid bodies. 1. In the intact rat, the O2-chemoreflex drive of ventilation controlled about 50% of the normoxic minute volume, and 85% in hypoxia. 2. Chronic bilateral carotid body denervation reduced the chemoreflex drive to half, and was accompanied by a hypoventilation with arterial hypercapnia. 3. In acute hypoxia hyperventilation was reduced in carotid-body denervated animals, and was accompanied by a light respiratory insufficiency. These results suggest that the rat has a powerful arterial chemoreflex drive of breathing which is essential in determining the eupneic level of ventilation in acute hypoxia.