Continuous positive-pressure breathing (CPPB) with an end-expiratory pressure of 10 cm H2O decreased cardiac output and urinary output, with an increase in plasma antidiuretic hormone (ADH). Similar results were obtained after bilateral cervical vagotomy. Oliguria during CPPB was caused neither by ADH release controlled by an intrathoracic volume receptor mechanism nor by changes in PaCO2, but was apparently related to the decrease in cardiac output Arterial oxygenation did not change during the periods of CPPB, but was increased upon release of the end-expiratory pressure. These changes in blood gases can be explained by the measured decrease in cardiac output or by an increase in pulmonary vascular shunting.