Changes in heart rate (HR) during muscular work under heat stress were used to establish work-rest cycles for heat acclimated subjects. Six male volunteers performed work loads of 40 and 60 percent of their maximum aerobic power (VO2max) in warm humid or hot dry environments. Work-rest periods were 20 minutes each for the 40 percent VO2max tests, and 10 and 20 minutes, respectively, for the 60 percent VO2max tests. Rest periods were in the same atmosphere as the work periods, or were in an air conditioned room. HR, rectal and skin temperatures, oxygen uptake, sweat production, and evaporative rate were measured. Work in the hot dry environment was less stressful than work in the warm humid environment for both work loads. HR and rectal temperature levelled off during successive work cycles in the hot dry condition regardless of the rest atmosphere, but this occurred for work in the warm humid condition only when rest was in the cool, air conditioned environment. The author concludes that cyclic work in hot environments for acclimated subjects can be designed based on the cumulative circulatory strain of the work load and the heat stresses. HR can be used as a strain indicator in relation to fractions of VO2max. When humid conditions are involved, resting periods must be scheduled in cooler neutral ambient conditions.