Temperature, humidity and radiant heat in an environmental test room were varied over wide limits while five heat stress indices were measured simultaneously: Effective Temperature, Effective Temperature Corrected for Radiation, Wet Bulb Globe Temperature, Heat Stress Index, and Wet Globe Temperature. Regression equations based on 34 tests for the five indices showed them to be interrelated at probabilities exceeding 99%. Similar tabulations were prepared from published data for other indices. An evaluation of the indices revealed that Wet Bulb Globe Temperature and Wet Globe Temperature are best suited to hot work situations where radiant energy is present. For hot, moist conditions, the Effective Temperature is the preferred index.