We examined the influence of physical load placed on high school baseball managers during midsummer games under extremely hot and humid conditions. The factors used to determine physical load were the following: body weight, oral temperature, amount of walking, pedometer count, heart rate, and serum biochemical elements. These factors were measured before and after the games. Twenty-two managers participated in this study. All games were played under high temperatures of 32.4 +/- 3.5 (mean +/- S.D) degrees Celsius dry-bulb, 27.1 +/- 3.0 degrees Celsius wet-bulb, 33.8 +/- 3.6 degrees Celsius black-glove, 29.1 +/- 3.3 degrees WBGT, which are likely to cause heart-related illness. The results were as follows. 1. After the games, significant body weight loss and oral temperature rise were found. Those findings were thought to be caused by the rise in oral temperature in a hot environment which was accompanied by hyperhidrosis. 2. The average hemoconcentration ratio based on the changes in total protein during the games was 105 percent, suggesting that hemoconcentration and dehydration were caused by sweating in a hot environment. 3. A significant increase in total protein, albumin, LDH, high density lipoprotein cholesterol, calcium, hemoglobin, and a decrease in triglyceride were observed after the games, which were thought to be influenced by sweating and by increased metabolism in a hot environment. 4. The values of triglyceride, Fe, uric nitrogen, calcium and hemoglobin after the games which were adjusted by the hemoconcentration ratio were significantly lower than those before the games. 5. A prolonged game time caused a significant increase in total protein value during the games and a decrease in hemoglobin between the level before the games and the adjusted level after the games compared with those values in the short game time group. From the above, even though high school baseball managers join in practices in a hot environment and become accustomed to it, we found that they had a great physical load on their bodies during the games in midsummer.