Oxygen saturation of arterial and central venous blood was measured continuously during the Valsalva maneuver in patients with and without intracardiac defects. The majority of patients with atrial septal defects and patients with ventricular septal defects and high right ventricular pressure showed a sharp decrease in saturation of radial artery blood shortly after the maneuver. These changes were not recorded accurately by ear oximetry. The patients with shunts uniformly showed a rapid increase in saturation of pulmonary artery blood during the initial period of increased airway pressure. This effect was not observed in patients without shunts. These reactions may be of special diagnostic value under certain circumstances.