The influences of an organofluoride (Nissol) poisoning on the heart were studied with respect to ECG of experimental rabbits and 5 intoxicated patients. 1. In the rabbits initially there appeared depression of ST segments and giant T waves, and terminally they succumbed to the sudden onset of ventricular flutter and fibrillation. 2. ECG from the patients were characterized by bradycardia, PQ prolongation, and U waves, but no arhythmia was seen. 3. These clinical ECG findings are compatible with vagotonia or hypopotassemia.