Clinical reports indicate that right ventricular diastolic collapse (RVDC) is associated with cardiac tamponade. To assess the hemodynamic abnormalities associated with RVDC, we studied six chronically instrumented conscious dogs (group A) with two-dimensional echocardiography during cardiac tamponade induced by continuous saline infusion into the pericardial space. We recorded cardiac output (electromagnetic flowmeter), heart rate, respiration, and aortic, intrapericardial, and right atrial pressures. In four additional animals (group B), we recorded right ventricular pressure and placed a hydraulic occluder around the pulmonary artery so that short-term reversible obstruction to right ventricular outflow could be produced. None of the dogs had RVDC when the pericardial space was empty, but all dogs showed RVDC during cardiac tamponade. The appearance of RVDC in group A was associated with a 21% reduction in cardiac output (p less than .01) and no change in mean aortic pressure. Short-term partial pulmonary artery obstruction led to increased right ventricular pressures and a striking reduction in RVDC in early tamponade, suggesting that RVDC is caused by pericardial pressure exceeding right ventricular pressure in early diastole. An additional animal had right ventricular hypertrophy caused by a severe "heart worm" infestation and did not show RVDC during cardiac tamponade. These observations suggest that in the absence of increased resistance to right ventricular outflow or right ventricular hypertrophy, RVDC occurs early in the course of cardiac tamponade and is associated with a hemodynamically important decrease in cardiac output.