We sought to document arterial oxygen saturation relative to changes in the right ventricular pressure/left ventricular pressure ratio (RVP/LVP ratio), an index of pulmonary arterial pressure, in infants born at high altitude. We performed pulse oximetry and echocardiography in 15 healthy infants born in Leadville, Colo. (3100 m), at 6 to 24 hours, 24 to 48 hours, 1 week, 2 months, and 4 months of age. Pulse oximetry was done under conditions of wakefulness, feeding, and active and quiet sleep. All infants received supplemental O2 at delivery and during postnatal transition; all oximetry measurements were performed with infants breathing room air. The mean arterial O2 saturation ranged from 80.6% +/- 5.3% to 91.1% +/- 1.7% during the 4 months. Values fell during the first week after birth and then rose gradually to attain near-birth values at 2 and 4 months of age. Arterial O2 saturation was uniform among behavioral states at 6 to 24 hours and 24 to 48 hours of age. After 1 week of age, values were highest during wakefulness, intermediate during feeding and active sleep, and lowest during quiet sleep. The RVP/LVP ratio remained in the normal or mildly elevated range throughout the study period. We conclude that the RVP/LVP ratio promptly becomes normal at high altitude, and despite low arterial O2 saturation in the first weeks to months after birth, healthy newborn infants at 3100 m show little evidence of acute pulmonary hypertension.