The effect of an increased ambient temperature on the incidence of acute myocardial necrosis caused by two subcutaneous injections of isoproterenol (80 mg/kg) 24 h apart has been investigated. Raising the temperature of the microclimate in the cages by 3–5 °C considerably increased the incidence of cardiac necrosis and the mortality after isoproterenol. Increasing the number of animals per cage enhanced the necrogenic effect of isoproterenol on heart muscle due to a rise in cage temperature. Exposing rats for 3–4 h after isoproterenol administration to a raised ambient temperature increased their sensitivity to isoproterenol. The same exposure made 20 h after isoproterenol administration had no such effect. Shaving the animals decreased their sensitivity to isoproterenol at an increased ambient temperature. The relation between the necrogenic effect of catecholamines and thermoregulation is discussed.