The effects of increasing ambient vapor pressure and air temperature on rectal temperature, mean skin temperature, and heart rate were assessed in eight heat acclimated, lightly clothed men and women under exercise stress. The subjects were acclimatized to daily treadmill walks under conditions of 50 degrees-C dry bulb (db), 25 degrees wet bulb (wb), or 45 degrees db, and 31 degrees wb prior to experimental measurements of heart rate, oxygen consumption, mean skin temperature, rectal temperature, and sweating made during treadmill exercise at speeds ranging from 1.34 to 1.56 meters per second at ambient air temperatures of 36, 38, 40, 44, 46, 48, and 52 degrees. Each test included 1 hour of fixed ambient vapor pressure followed by a second hour of increasing vapor pressure, and treadmill speeds were adjusted to yield similar metabolic heat production for all subjects. The rectal temperature, mean skin temperature, and heart rate reached steady state levels within the first hour. No correlation was determined between the equilibrium rectal temperature and the ambient air temperature, and no significant differences were determined between rectal temperatures on the basis of gender. Increasing the ambient vapor pressure during the second hour produced subsequent increases in all three physiological responses, and significant gender differences were determined for heart rate but not for skin or rectal temperatures.