We studied whether maternal acid-base status during aerobic exercise performed at moderate altitude is affected by pregnancy. Seven primiparus women were tested at 37 wk gestation and 12 wk postpartum. Subjects were studied at rest, and during two cycle (50 W, 75 W) and two treadmill (67 m.min-1; 2.5% grade, 67 m.min-1; 12% grade) protocols. Exercise bouts lasted 6 min with a 10-min rest between sessions. Minute (VE) and alveolar (VA) ventilation, tidal volume (VT), and ventilatory equivalent for carbon dioxide (VE/VCO2) were significantly (P less than 0.01) greater when exercise was performed during pregnancy. Physiological dead space (VD) was not affected by pregnancy status and did not differ between rest and exercise. Decreases (P less than 0.01) in arterial pH during exercise averaged 0.04 units in both pregnancy and postpartum. Despite similar change in maternal pH, carbon dioxide tension (PaCO2) remained unchanged during exercise at 37 wk gestation but decreased at 12 wk postpartum. Decreases in arterial bicarbonate [HCO3-] associated with exercise were smaller during pregnancy. Our findings indicate that pregnancy did not compromise maternal acid-base status during aerobic exercise.