We studied the changes in PO2 during 72 isolated episodes of apnea of prematurity in 20 low-birth-weight infants. A stable PO2 prior to the onset of apnea was observed in 65% of the episodes. A falling PO2 was noted in only 18% of the cases. The mean initial PO2 was 75 mm Hg and no infant was hypoxic (PO2less than 40 mm Hg) immediately prior to apnea. The mean PO2 fell to 60 mm Hg at the end of apnea and continued to fall to a mean low PO2 of 46 mm Hg. The mean PO2 at recovery was 79 mm Hg. We concluded that in this group of premature infants, hypoxia is not the initiating even in the apnea of prematurity. Furthermore, we noted that arterial oxygen tension continues to fall despite the reestablishment of respiratory efforts, that the rate of recovery from apnea is slower than the rate of fall, and that bradycardia associated with apnea is not initiated by hypoxemia.