Past attempts to devise an index of climatic stress are briefly reviewed, and a new “Relative Strain Index (RSI)” is proposed. RSI is the ratio between the evaporative cooling required to compensate for the heat stress, and the maximum evaporative cooling that the animal can provide by physiological means. RSI takes into account the air temperature, vapor pressure, air movement,and radiant heat exchange,as well as the metabolic rate and insulation provided by the animal's coat. The index can be used to predict the probable tolerance of animals of types that have been sufficiently well studied. It can also be used to estimate the relative tolerances of animals having different physiological capacities or metabolic rates, but otherwise comparable; and it can be used to predict the relative effectiveness of proposed environmental controls.