Three subjects walked continuously on a treadmill for periods of 1 hr or more at 180, 300, or 420 kcal/hr in a range of cool and hot climates from corrected effective temperature (CET) 10–32 C. At each rate of work rectal thermal equilibrium was practically independent of the influence of environment over a wide range of climates (“prescriptive” zone); the upper limit of the prescriptive zone appeared to be associated with the minimal bodily thermal gradient compatible with the transfer of adequate amounts of heat from the core to the periphery without placing the thermoregulatory system under disproportionately increased strain, in terms of circulatory response and elevated body core temperature; as such, this seems to be one possible criterion by which thermal environmental limits for everyday work may be assessed. The upper limits of prescriptive zones for work at an energy expenditure of 300 kcal/hr is CET 27.4 C and those for the lower and higher rates of work, respectively, were CET 30.2 C and 26.9 C. Submitted on June 15, 1962