A study was carried out on three groups of 20 male subjects to determine the combination of environmental stress and a standard rate of work of 5 kcal/min that will give the “optimum” level of acclimatisation. Each group was acclimatised at one of three temperature conditions (wet-bulb temperatures of 32.2 °, 33.9 ° and 35.6 ° C) and thereafter tested also under the remaining two conditions. The degree of acclimatisation was judged by the physiological reactions to a standard work rate of 5 kcal/min at the various wet-bulb temperatures. The test at a wet-bulb temperature of 32.2 “C revealed that °optimum” acclimatisation was obtained when men were acclimatised at 33.9 °C W.B. The test at 33.9 °C W.B. illustrated that the physiological reactions of the group acclimatised at 35.6 ° C W.B. were decidedly poor when compared to those of the groups acclimatised at 32.2 ° and 33.9 ° C W.B. The test carried out at 35.6 ° C W.B. showed that, irrespective of the wet-bulb temperature at which the men were acclimatised, their heat tolerance to the test environment was poor.