Cardiovascular effects of denopamine (TA-064), a new positive inotropic agent, in chronically instrumented dogs were investigated following intravenous and oral administration. In the conscious state, denopamine (0.5-4 μg/kg/min, i.v. infusion) increased LV dp/dt_max, cardiac output, stroke volume in a dose-dependent manner, and it decreased left ventricular end-diastolic pressure, total peripheral resistance and PQ interval. Denopamine produced an increase in LV dp/dt_max by 90% at a rate of 4 μg/kg/min with slightly increasing systemic blood pressure and heart rate. In the same dogs after anesthetizing with pentobarbital, denopamine in the same dose range showed more marked positive inotropic effects and less changes in cardiac output and total peripheral resistance than in conscious dogs. Other cardiovascular effects of denopamine were qualitatively similar to conscious dogs. These effects of denopamine were diminished by treatment with propranolol. Oral administration of denopamine to conscious dogs (0.1-0.4 mg/kg) produced a rise in LV dp/dt_max dose-dependently, and denopamine at a dose of 0.4 mg/kg increased LV dp/dt_max by 66%, this effect lasting for 7 hr. Heart rate and blood pressure were not affected significantly. Effects on other cardiovascular parameters were changed in the same direction as intravenous administration. The increase in LV dp/dt_max corresponded well with the changes in plasma levels of denopamine in conscious dogs by both intravenous and oral administration. Denopamine showed a selective positive inotropic effect in chronically instrumented dogs, and its positive inotropic action was more marked in the myocardium depressed with an anesthetizing dose of pentobarbital than in the conscious state.