Metabolic rates were compared on seven young men performing three levels of physical activity at three environmental temperatures of 70, 85, and 100 F. This study indicates that as the environmental temperature increases there is also an increase in metabolic rate of men performing a fixed activity. It has been shown that there was a significantly higher metabolic rate for men working at 100 F than at 85 and 70 F. These increases averaged 11.4% for the rest period, 13.3% for the moderate activity, and 11.7% for the heavier activity. Body temperatures also were significantly higher at 100 F than at 85 and 70 F environments (P < .005). They averaged 99.6 F for the 100 F temperature, and 99.1 F for both the 85 and 70 F temperatures. The findings in this study indicate that the metabolic rate of a fixed physical activity is increased in the heat and that this increase is not due to acclimatization or training. Submitted on September 19, 1962