Crystalline type A botulinal toxin rapidly caused temporary bradycardia and electrocardiographic (ECG) changes in mice, rats, rabbits and dogs. In addition, in the dog the force of contraction was measured and found to be depressed. The ECG changes were indicative of conduction defects. The hemagglutinin present in the toxin played no role in the effects on the heart, since a derivative toxin without hemagglutinin also caused these phenomena. The cardiac effects were spontaneously reversible in the intact animal without removal of the toxin. On the other hand, in the in vitro isolated heart of the rat, recovery from the cardiac effects occurred only after the toxin was washed out of the preparation. The findings are consistent with, but do not prove, a physical rather than a chemical mechanism for the effects of toxin on the heart.