This report describes a study by Dr. Farber and colleagues to investigate the effect of carbon monoxide (CO) exposure on cardiac parameters in dogs. They tested dogs that developed ventricular fibrillation during a test of exercise and acute myocardial ischemia (i.e. that were susceptible) and dogs that survived the test without fatal arrhythmias (i.e that were resistant). Susceptible and resistant dogs were exposed to a concentration of CO sufficient to raise carboxyhemoglobin levels to 5 – 15%. Heart rate, bradycardia, and ventricular arrhythmia were then monitored at rest or during exercise.