Effects of 10% O2 on the response of the phrenic neurogram to vagal (VS) and saphenous stimulation (SS) were studied in anesthetized and artificially ventilated newborn (group 1: 1-3 days old, n = 13; group 2: 8-14 days old, n = 14) and adult (group 3, n = 12) rabbits. In all animals during ventilation with 100% O2 or 10% O2 in N2, VS caused a reduction in the expiratory phase (Te). Te shortening in response to SS was unaffected by hypoxia in group 3, and was abolished in group 2. In group 1, only early hypoxia blocked the Te response to SS. The preserved Te response to VS on one hand, and abolished the effects of SS on the other, suggest that blockade of the somatic inputs is due to the age-dependent action of hypoxia in areas distant to the respiratory neurons.