We evaluated the hypothesis that a 'central oxygen detector' in the brainstem is necessary for depressions of ventilatory activity to be manifested in the newborn. Decerebrate piglets, ventilated with 100 % O(2), were studied following neuromuscular blockade. The vagi and carotid sinus nerves were sectioned bilaterally in order to remove the influence of the peripheral chemoreceptors. Activity of the phrenic nerve was recorded as the index of the central respiratory rhythm. This activity declined and, in some preparations, ceased upon ventilation with air or a hypoxic gas, at either normocapnia or hypercapnia. The degree of depression in hypercapnic hypoxia was greatest in the youngest piglets. Following a medial section of the brainstem, extending to the caudal pons, the depression was reduced. In some preparations, a similar reduction followed the placement of radiofrequency lesions in the caudal ventromedial pons. We conclude that a region of the caudal mesencephalon or pons is necessary for the manifestation of depressions of ventilatory activity in the newborn pig.