The effects of rest breaks and type of clothing on tolerance to extreme heat were assessed in 12 heat acclimatized men intermittently exposed to heat for periods of 8 hours. The conditions of exposure included ambient air temperatures of 50, 40, 35, and 30 degrees-C dry bulb; work rest intervals of 90 percent work and 10 percent rest, 78 percent work and 22 percent rest, and 67 percent work and 33 percent rest; and industrial clothing outfits consisting of a light weight shortsleeved shirt and trousers, a jumpsuit, and a medium weight longsleeved shirt and trousers. Metabolic rate was controlled by using a Masters' step task at a rate of ten trips per minute for 2 minutes followed by 3 minutes of standing. The variables measured during the test included heart rate, blood pressure, skin temperature, rectal temperature, weight loss, and subjective opinion. Significant correlations were established between the environmental temperature and the rectal temperature, skin temperature, heart rate, and subjective thermal comfort and sensation at all work rest intervals. The most important criterion for heat tolerance was the percent time worked, and the least important criterion was the rise in rectal temperature. The results were discussed in relation to the appropriate index of heat stress for a working environment, the benefits of cool rest areas versus warm rest areas, and the thermal comfort of different clothing ensembles.