The relationship between free heart rate and dP/dt max in the left ventricle induced by isoprenaline in the dog was changed by propranolol so that, for a given free heart rate, dP/dt max measured less in the presence of propranolol than in its absence. This effect of propranolol was not present in dogs previously depleted of catecholamines by treatment with reserpine. Propranolol and catecholamine depletion reduced the control dP/dt max by about 800 mm Hg/sec with no effect on the heart rate. It is concluded that propranolol (given intravenously) is a pure β-adrenoreceptor blocking drug unless the dose exceeds 1-2 mg/kg.