The effects of administering atmospheres containing 10 and 12% oxygen to healthy newborn infants of different ages have been studied. Infants under 24 hours of age tended to hypoventilate throughout the period of hypoxia. Infants six to 11 days of age hyperventilated for the first two or three minutes and then showed a decrease, although not so marked as observed in the infants under 24 hours of age. Infants 16 to 48 days of age showed the most marked increase in ventilation, but even in these infants it was poorly maintained as compared to responses seen in adults. Hypoxia produced a slowing of the respiratory rate in all infants except the very oldest. Tidal air was increased at first and then decreased. The increases in tidal air accounted for most of the increase in minute volume seen in the older infants. Hypoxia increased the incidence of periodic breathing in the two older groups of infants, but had very little effect on infants under 24 hours of age. Respiratory patterns did not appear to be altered by the hypoxic conditions employed in this study. It has been postulated that the comparatively weak response to hypoxia made by infants immediately after birth possibly is dependent on relatively poor chemoreceptor reflexes and that the latter increase in strength during the first weeks of postnatal life. It has also been suggested that the relative inability to induce periodic breathing in infants under 24 hours of age as compared to those several weeks old is further evidence indicating that metabolism of the medullary respiratory centers is to a larger extent anerobic than at the later periods of postnatal life.