1. Slowing of the heart occurred in monkeys and guinea pigs during the febrile period of the experimental infection due to Leptospira icteroides. A similar reaction took place in animals inoculated with Leptospira icterohœmorrhagiœ. 2. The mechanism of slowing was usually due to slowing of the whole heart. 3. Once incomplete heart block was seen. Changes in the ventricular complex occurred four times.