Positive inotropic responses, measured as increases in the maximum derivative of left ventricular pressure (dP/dt), were induced by carotid artery occlusion (CAR), intravenous norepinephrine (NOR), or intravenous isoproterenol (ISO) in the canine heart in vivo. NOR and ISO also increased peak ascending flow velocity (V max). ISO increased heart rat (HR), while NOR and CAR produced comparatively little chronotropic response. Phentolamine (1 mg/kg) considerably reduced induced increases in dP/dt, Vmax and HR, while the same dose of propranolol almost completely abolished such increases. The results indicate that phentolamine, and alpha-adrenergic blocking drug, does have and ability to partially block induced positive inotropic responses in the canine heart, and perhaps chronotropic responses as well, although propranolol, a beta-blocker, is more effective. the results are in accord with the view that the principal cardiac adrenergic receptors are not adequately described either as classical beta- or alpha-adrenergic receptors.