The performance of physical work in the heat can be affected for better or worse by a variety of factors, the most important being physical fitness and acclimatization, nutritional state, clothing and drugs. This paper deals with the effects of water, salt and glucose, with principal emphasis upon the attainment of the best possible performance. The effects of water soluble vitamins will be mentioned briefly. The present material will be restricted in two ways. First,. attention will be confined to performance of fixed tasks by fully acclimatized men working intermittently in moist or in dry heat. Therefore, the results are applicable to steel workers, miners and soldiers in tlhe desert, all of whom usually do a day's work in the heat and spend the rest of the time in cooler surroundings. Second, we shall consider only the immediate hour to hour effects of the above dietary factors upon men n-hose overall daily intake was always adequate in water, salt, carbohydrate and vitamins. There will be no consideration of prolonged deficiency or excess from day to day. METHODS. Experiments were performed in the late fall or winter in a heated room under hot dry (100 F., 30 per cent relative humidity) and hot moist conditions (95 F. and 90 F., 83 per cent relative humidity). Six healthy young men, fully acclimatized as judged by the criteria of Robinson and colleagues (1943), marched at least three times a week at 3.5 m.p.h. up grades which will be specified in the tables. Depending upon the severity of the temperature and humidity, which were const,ant in individual series of experiments, they marched anywhere from one to six hours with a ten minute rest in each hour. Certain measurements were made periodically in every experiment. These were: a, environmental temperature and humidity with a sling psychrometer; 6, pulse rate by palpation; c, rectal t#emperature with a calibrated clinical thermometer; d, sweating by net change in nude body weight. Other measurements were made in some, but not all, experiments. These were: e, respiratory exchange by collection of expired air in a Tissot gasometer after the subject was in a steady state, analysis for carbon dioxide and oxygen with the Haldane apparatus, and calculation of the oxygen consumption by standard procedures (Haldane, 1934; Peters and Van Slyke, 1932); f, protein by the micro Kjeldahl method of Ma
