The influence of different levels of water deficit on the physiological responses to heat stress of two well-acclimatised subjects was studied. The subjects worked continuously at a rate of 2,000 ft.lb/min for four hours at 32.2 ° C W. B., 33.9 ° C D.B. and an air velocity of 0.25–0.4 m/sec. Man, even when in water deficit, operates most efficiently when he replaces all the fluids lost in sweat and urine by drinking water in small amounts at frequent intervals. When a specific level of water deficit is maintained throughout the 4 hours of heat exposure body temperature, heart rate, and sweat rate reach equilibrium at values which are significantly different from those recorded under conditions of complete water balance; the more severe the level of dehydration the higher the body temperatures and heart rates, and the lower the sweat rates. No indication of any failure of the temperature regulating mechanisms or fatigue of the sweat glands was found and a possible explanation for this difference with previously reported results is provided.