To obtain normal data on arterial oxygen saturation as measured by pulse oximetry (SpO2; Nellcor N200), we obtained 12-hour tape recordings of SpO2, photoplethysmographic waveforms, instantaneous pulse rate, and observations of breathing movements on 55 preterm neonates (25 girls) who had been admitted to one of four special care baby units but had no signs of respiratory distress and were breathing room air at 24 hours of age. Their median gestational age at birth was 35 weeks (range, 30 to 36), and their median age at the time of study 1 day (range, 1 to 7). Median baseline SpO2, measured only during regular breathing, was 99.4% (range, 90.7 to 100; 5th percentile, 95.5). Ten recordings (18%) contained a total of 83 episodes of desaturation (defined as a fall in SpO2 to < or = 80% for > or = 4 seconds). The 95th percentile for desaturation frequency was eight per recording. One infant had 55 episodes of desaturation and thus accounted for two thirds of all episodes observed. Only one of the episodes of desaturation in this infant, and none of those in the other nine infants, had been noted clinically, nor had the abnormally low baseline SpO2 (90.7%) in one infant. Baseline SpO2 in these nondistressed preterm neonates was higher than might be expected, given the SpO2 levels currently recommended for preterm infants with respiratory failure. A minority of infants, however, had a low baseline SpO2 or a high frequency of episodes of desaturation, the potential effects of which remain to be determined.