Baseline hemodynamic variables were established during morphine - N2O-O2 anesthesia in dogs following which N2O was discontinued and halothane 2% (end-tidal) in O2 was administered. Halothane caused significant decreases of 58, 55, and 66 percent, respectively, in cardiac output, mean arterial pressure, and left ventricular dP/dt and significantly increased pulmonary vascular resistance (161%) when compared to measurements obtained under morphine - N2O-O2 anesthesia. During halothane anesthesia, amrinone, isoproterenol, and dobutamine were administered by continuous intravenous infusion in doses which restored cardiac output to pre-halothane values. Left ventricular dP/dt was increased and pulmonary resistance was decreased significantly by the three positive inotropic agents. Isoproterenol and amrinone significantly decreased systemic vascular resistance 48 and 71 percent, respectively below the value for halothane alone. Dobutamine significantly increased mean arterial pressure. Heart rate and left ventricular end-diastolic pressure were not altered by any treatment. Ventricular arrhythmias were associated with the infusion of all three agents. It is concluded that amrinone, isoproterenol, and dobutamine were capable of restoring cardiac output which had been decreased by halothane. However, isoproterenol and amrinone caused marked decreases in systemic vascular resistance which precluded an increase in arterial pressure above the value encountered with halothane alone.