Inotropic support for the failing myocardium as the therapy for xi congestive heart failure (CHF) is intended to achieve an increase in cardiac output via positive responses in myocardial contractility and vasodilation. A novel approach to differentiate these two responses is the use of an animal with an implanted total artificial heart (TAH). Three inotropic drugs, dobutamine, enoximone, and pimobendan, were tested in eight animals with their natural heart intact and five animals implanted with a TAH. Baseline values of the TAH and natural heart (NH) were compared to determine their hemodynamic similarities. Each of the three drugs was given randomly to the animals in dosages similar to human clinical doses. Peak responses were recorded and analyzed. All three drugs caused an increase in contractility and cardiac output in the NH animals. Dobutamine and pimobendan also caused a significant increase in heart rate at higher dosages whereas enoximone did not. Dobutamine caused an increase in left ventricle work, as did pimobendan at the first dose given; at higher doses of pimobendan, the left ventricular work returned to baseline. However, at the doses tested, the left ventricular stroke work during enoximone administration decreased. Vasodilation (the only drug stimulation response in the TAH model) was also observed with the administration of the drugs in T AH animals, and all three caused decreases in systemic and pulmonary vascular resistance. Dobutamine and pimobendan caused an increase in left and right atrial pressures (because of the mechanical heart not being adjusted to compensate the increased return). There was also a reduction in systemic and pulmonary resistance. Enoximone caused severe pulmonary hypertension in the TAH animals, possibly due to stimulus of platelets to release vasoconstrictive substances. Thus, dobutamine, enoximone, and pimobendan significantly contribute to increases in output by vasodilation in animals with a natural heart. Similarly, dobutamine and pimobendan's vasodilatory action is identified in an animal with a TAH. However, enoximone's hypertensive action on the pulmonary vasculature of a TAH animal may offer an insight to the toxicity of enoximone when used after recent surgery.