Ethaverine inhibits both atrioventricular conduction and heart rate, starting at a dosage of 1 mg/kg, injected i.v. into the anaesthetized dog. In the experimental conditions used, ethaverine is clearly different from papaverine, which stimulates both at a dosage between 1 and 10 mg/kg, and which only produces an inhibitory effect starting at a dosage of 20 mg/kg, with the appearance of cardiotoxic phenomena. After total cardiac denervation the inhibitory action of ethaverine persists partially, especially at the atrioventricular level, which is probably due to a direct effect on the heart muscle. The observed differences can hardly be explained by a more pronounced cardiac toxicity. Consequently, the ethyl-derivative of papaverine constitutes, in certain aspects, a molecule with original pharmacological properties.