The prescriptive zone is the range of climates in which man's body temperature is independent of climatic conditions. The environmental temperatures which define the upper limit of the prescriptive zone (ULPZ) at different work rates were determined in 46 clothed, healthy, male industrial workers; some of the men were job acclimated to heat and some were not. They performed a total of 653 work bouts of low, medium, or high intensity in environments ranging from 11 to 35 degrees C corrected effective temperature (CET) (8-37 degrees C wet bulb globe temperature). Heart rates (HR) and rectal temperatures (Tre) were measured after 1 h of work. The ULPZ was calculated from Tre data. HR's showed a similar pattern of response as Tre's except that the inflection point corresponding to the ULPZ occurred at different environmental temperatures at most of the experiment conditions. About one-third of the work bouts were performed in the summer months and the remainder in the winter. The ULPZ decreased with increasing work rates. At high, but not low, work rates, men who were exposed to heat in the performance of their jobs were more heat tolerant than men who were not heat acclimatized. Both groups were found to be more heat tolerant in the summer months than in the winter.