An experimental study was made on the changes of physiological functions and the dynamic aspects of thiamine metabolism caused by mental work under hot atmospheric stimulation for a relatively long time. The experimental subjects were 3 healthy men of 20-24 years old. Daily fluctuations in the total thiamine content of blood were observed even under normal conditions, and the content seemed to depend on the amount of the thiamine intake. After oral administration of thiamine, an increase of the level of free thiamine of the blood plasma occurred. This was soon followed by a rise of the level of esterified thiamine of the whole blood. Eventually, both gradually decreased and reached their original levels. Under hot atmospheric conditions for a relatively short time, the level of thiamine of blood was lowered, and free thiamine was not observed in the blood plasma. When a heat stress over the critical temperature (temperature at which a sudden rise of pulse over 100 is observed) was applied to a subject who had been subjected for 2-3 hours to a preliminary stress nearly at this temperature, a marked rise of the total thiamine level of blood was observed in at least 7-8 hours after the preliminary stress. Since no free thiamine was found in blood at this period, it seemed that the rise of the total thiamine level was due to the increase of the esterified form in the plasma. The free thiamine content of the urine increased during this period. When the stress was removed a marked increase was observed. Although seasonal variation seemed to play a role, the exact reason for this phenomenon has not yet been clarified. It was also found that the second heat stress exerted an inhibitory effect on the increase of free thiamine level.