The experiments described in this paper include observations of physiological and subjective responses, and performance of mathematical computations under different climatic conditions. The subjects underwent psychological tests and the correlation between their intelligence and emotional stability to their performance was chocked. A now thermal index wag compared with the ‘ Effective Temperature ’ and found in bettor agreement with the results of the physiological investigations. The results suggest that it is possible to maintain the maximal degree of mental ability in hot environments for short periods, provided that high incentives are offered