The hemodynamic effects of isoproterenol infusion, 0.5 mug/kg per min were evaluated in eight intact anesthetized dogs during cardiac tamponade. During tamponade, the mean of pericardial pressures was increased from - 1.5 to 12.5 mm Hg, and the mean of right atrial pressures was increased from 1 to 12.4 mm Hg. Mean cardiac output fell from 144.8 to 44.8 ml/kg per min (P < 0.001), and rose to 105.6 ml/kg per min (P < 0.001) with isoproterenol. Mean cardiac stroke volume fell from 20.3 to 6.1 ml during tamponade (P < 0.001) and rose to 12.1 ml with isoproterenol (P < 0.001). The heart rate increased from 193.3 beats/min during tamponade to 217.5 beats/min with isoproterenol (P < 0.05). During isoproterenol infusion, the mean right atrial pressure and mean pericardial pressure decreased significantly. With cardiac tamponade, the mean blood pressure fell from 157.5 to 126.1 mm Hg (P < 0.01) and did not change significantly with isoproterenol, 11 additional animals were studied with norepinephrine infusion during tamponade. There were no consistent hemodynamic effects with infusions of 0.5 and 1 mug/kg per min. With norepinephrine 2, 5, and 10 mug/kg per min cardiac output rose in some experiments. Isoproterenol infusion increased the cardiac output during tamponade principally by increasing cardiac stroke volume and to a lesser degree by increasing the heart rate. It is postulated that the increased stroke volume resulted from an increased ejection fraction with greater decrease in end-systolic than end-diastolic ventricular volume. These effects are consistent with the known positive inotropic, peripheral vasodilator, and positive chronotropic effects of isoproterenol.