Often workers exposed to heat show more than 1.3l perspiration per an hour. Plasma condensation is slight and the recovery is quick by trained persons. By untrained persons plasms condensation is more than that of Hb and its restoration is delayed. When plasma is condensed more than 1.034 in its specific gravity, creatinine excretion in the urine decreases unless water is taken. Creatinine hardly responds to the urine volume for its difficulty of reabsorption in the urinary tubule until the volume is reduced to less than 20 ml/hr, and after that creatinine excretion decreases very much. Na, Cl and K diminish in accordance with the urine volume as they are reabsobed in the urinary tubule. The relation among them can be determine if creatinine is employed as an indicator. The constituent of sweat is affected by the perspiration rate. In the course of excessive perspiration pH becomes alkali and lactic acid pyruvic acid ratio increases. PH becomes acidic and Cl concentration is low in summer, both of which are related to the perspiration rate (figs. 1, 2). It is possible to recognize the summer-type of swest even in winter by using DOCA. Ig DOCA is used in summer, it causes Na retention and cardiac disfunction. Water and salt are deprived by excessive sweating as already known. Ca concentration in the sweat is a little smaller than in the serum. 6l perspiration a day means a loss of 0.4 gr Ca a day. It is very important, for the Japanese, because they take very small amount of Ca (0.3-0.4gr a day). The first function that limits work in hot environment is pulse rate. 130 beats per minute is thought to be the limit. It is difficult to continue the work with the pulse rate of 160-180. Workers of long experience in hat environment complain of the heart trouble, which will probably be the result of their cardiac burden and overfunction of the adrenal cortex. Fatigue in the excessive sweating appears to be due to the overload in circulatory systems and the loss or anormal distribution of extracellar fluid and minerals by sweating. Intake of the salt of organic acid such as sodium citrate was useful for diminishing the feeling of fatigue and maintaining alkali equilibrium. Taking tablets of table salt does injure the mucous membrane of the stomach.