Papaverine is an opium alkaloid of the benzylisoquinoline group, of low toxicity and non-habit forming.1At the present time its widest clinical use is in the treatment of conditions associated with smooth muscle spasm, e. g. peripheral vascular embolism,2pulmonary embolism,3Raynaud's disease4and ureteral5or other types of tubular spasm. We have recently investigated its action on the dog's heart and have found that it is a powerful and lasting coronary vasodilator,6that it diminishes considerably the ease with which ventricular fibrillation is induced by faradic or other type of stimulation7and that after its administration vigorous manual massage of the heart will restore orderly, synergic beating to the fibrillating ventricles.8 Further studies on the dog's heart9revealed that: 1. Papaverine in large doses depresses auriculoventricular and intraventricular conductivity leading to auriculoventricular and intraventricular block. 2. It decreases or eliminates