Using M-mode and two-dimensional echocardiography, we have compared left and right ventricular dimensions at expiration and inspiration in a group of 12 healthy young volunteers during the following two distinct periods: 1) a control period with atmospheric pressure breathing, and 2) a continuous positive airway pressure (CPAP) period with 15 cmH2O end-expiratory pressure breathing. In 6 of the 12 subjects we also evaluated inferior vena caval size, using the same technique. Inspiratory decrease in left ventricular short-axis area (A), length (L), calculated volumes (V = 5/6 AL), and inspiratory increase in right ventricular short-axis diameter and long-axis area were evident at both control and CPAP periods. CPAP also produced a marked decrease in left and an increase in right ventricular dimensions during both expiration and inspiration and a significant decrease in calculated stroke output. An increase in vena caval size during CPAP breathing indirectly suggested a decrease in venous return, but the finding of an unchanged percent of inspiratory change of this vessel during CPAP indicated that the inspiratory augmentation of venous return was still present. On the other hand, the finding of an increased right ventricular size indicated that the right ventricle was afterloaded by CPAP; this probably could explain the observed reduction in calculated stroke output.