SUMMARTMuscle blood flov remained unchanged during pro'longed exercise when pigs were cooled by skin wetting and a fan. This muscle blood flov response was comparable to that of humans exercising under similar conditions. The effect of temperature on this response i's uncertain-since both increases and decreases in muscle blood flow have been reported during thermal stress.Svine were similar to humans in regulatinj blood flow distribution during prolonged exercise.Cardiovascular drift in swine differs from that in humans due to difference in temperature regulation, posture and maintenance of central blood volume. The upward drift in heart rate and cardiac output -during prolonged exercise was probably the result of increasing skin blood flow and sympathetic drive. Upward drift in oxygen consumption in animals has not been well demonstrated, but our results tend to support the known contributing factors in humans.' Further work is needed to determine the role of temperature and exercise intensity on the mu.cle blood flow response to prolonged exercise.