The Australian freshwater crocodile, Crocodylus johnstoni, heats significantly faster than it cools. Under the experimental conditions, cooling rate was found to be approximately 80% of heating rate in both air and water. Heart rate during heating is more rapid than during cooling. Convective transfer of heat within the body is effected by the circulatory system, and this far exceeds rates of heat transfer by conduction alone. Localized heating of the body surface results in a cutaneous vasodilation which promotes uptake of heat. Localized cooling of the body surface results in cutaneous vasoconstriction, promoting heat conservation. Increase or decrease in heart rate accompanies cutaneous vasodilation or vasoconstriction. The dilation or constriction of cutaneous blood vessels and the acceleration or deceleration of heart rate are seen as separate responses to surface heating or cooling. Together they promote heat gain or retard heat loss and provide physiological mechanisms contributing to thermoregulation in C. johnstoni.