van Rensburg, A. J., Mitchell, D., van der Walt, W. H., and Strydom, N. B. (1972).Brit. J. industr. Med.,29, 387-393. Physiological reactions of men using microclimate cooling in hot humid environments. This paper describes the laboratory testing of a water-cooled vest and of a pre-frozen jacket on men working in hot humid environments. The work rate used in the tests was comparable with that of moderately hard industrial work. Three measures of strain were used, namely, rectal temperature, heart rate, and sweat rate. The tests showed that the water-cooled vest provided protection physiologically equivalent to removing the entire environmental heat stress, even at wet bulb temperatures of 33·9°C. The pre-frozen jackets provided better protection at 32·2°C wet bulb than at 33·9°C wet bulb. The garments have the potential to restore the loss in productivity caused by heat stress.