Twenty-three nonsmoking pregnant women ranging in age from 16 to 34 years and free of any signs or symptoms of pulmonary disease were evaluated by arterial blood gas values, while both sitting and supine, and closing volumes (CV) within 6 weeks of delivery and in the immediate postpartum period. Arterial oxygen tensions are usually normal while sitting, but modest hypoxemia occurs in about 25% of healthy young women while supine. The calculated alveolar- arterial gradient (A-a O2) was generally abnormal in the supine position, and in 11 of 23 women was greater than 20 mmHg. This abnormality persisted in the immediate postpartum period. Closing volumes fell significantly after delivery, but there was no significant relationship between changes in closing volumes and the arterial oxygen tension or the A-a O2. The presence of an elevated A-a O2 is not helpful in the assessment of possible pulmonary embolism in this population. To avoid overdiagnosis of pulmonary embolism, more attention should be given to the actual value of the arterial Po2, which should be drawn while the patient is in the upright position.