ABSTRACT. Repetitive polysomnograms were recorded from a total of 33 infants, 19 healthy preterm infants, and 14 term controls between 40 wk postconceptional age and 6 mo of age. These nighttime recordings lasted 2-4 h, except at 52 wk in preterm infants and at 3 mo of age in term infants when an overnight 12-h recording was per With the onset of pulmonary ventilation after birth, arterial Po, levels rapidly rise from the 2.7-4.1 kPa (20-30 torr) characteristic of the fetus to approximately 8.1-9.5 kPa (60-70 torr), but Pco, levels do not change dramatically (1). During the first weeks of life, PCO, sensitivity increases slowly and P C O~ threshold decreases with age as evidenced by a displacement of the PCO,