This study evaluated the effects of labor pain on maternal arterial hemoglobin oxygen saturation and neonatal acid-base status. Arterial oxygen saturation was monitored noninvasively by pulse oximetry during labor and delivery. The patients studied (N = 46) were divided into four groups according to obstetric history (primiparas and multiparas) and pain management during labor (lumbar peridural anesthesia versus meperidine and nitrous oxide). Nine patients at term but not in labor served as controls. Decreases of arterial oxygen saturation were related to both subjective pain, reported by visual pain analog scales, and to neonatal acid-base status at delivery. All values are reported as mean +/- standard deviation (SD). Primiparas with peridural anesthesia showed significantly less decrease in arterial oxygen saturation (1.7 +/- 1.4%; P less than .001; N = 15), superior scores on the visual pain analog scale (3.5 +/- 2.0), and a significantly better neonatal acid-base status (pH 7.29 +/- 0.06; P = .01; base excess -6.4 +/- 2.2; P less than .05) as compared with primiparas treated with meperidine and nitrous oxide (SaO2 7.2 +/- 3.9%; visual pain analog scale 7.1 +/- 1.2; pH 7.21 +/- 0.1; base excess -9.5 +/- 4.5; N = 16). In multiparas there was no statistically significant difference in decrease of arterial oxygen saturation, visual pain analog scale, and neonatal acid-base status.