With the purpose to determine the influences of hot and highly humid environment on the B(1)-metabolism the author pursued the changes in the tissue B(1)-eontent and body weight in albino rats fed various foods for 30 days under a fixed hot environment and obtained the following results. 1. The concentration and quantity of the tissue B1 in the albino raised with common foods under hot environment decreased temporarily 5 days after the start of experiment, but thereafter with the lapse of time the B(1)-concentration gia ally re ed to the normal level. However, the absolute B1-content in the liver alone remained at a low level even 30 days later. 2. When young albino rats were raised with Shimazono B(1)-deficient diet or with low fat Shimazono B(1)-deficient diet for 30 days under hot environment, the decrease in the tissue B(1)-concentration showed no significant difference from that in the control group raised under the normal environment. Howeyer, the B(1)-content in the liver of all those raised under the hot environment alone showed a lower value than in the control group. 3. The tissue B(1)-concentration and quantity in the young albino rats raised with the low fat diet under the hot environment showed markedly lower values than those observed in the group raised with the B1-deficient Shimazono diet. 4. The gain in the body weight of the albino rats raised with various foods under hot environment was markedly poorer than that in the control group raised under the normal environments. However, the loss in the body weight of the young albino rats fed B(1)-deficient diet was less in the group raised under the hot environment than that in the control raised under the normal temperature. 5. The fluctuations in the B(1)-content and concentration of the brain were remarkably small throughout the experiment in both environmental conditions when compared with those of other tissues.