This study attempted to measure the physical load on national high school baseball umpires during games played at Koshien stadium under extremely hot and humid conditions in the summer. Thirty-one umpires participated in this study. Thirteen of them were evaluated twice while eighteen were evaluated only once. The factors used to determine physical load were the following: body weight, oral temperature, blood pressure, heart rate, and serum biochemical elements. These were measured before and after the games. Heart rate was measured at one-minute intervals. The results were as follows. 1) All the games were played under conditions of extremely high temperatures--32.1 degrees celsius dry-bulb, 27.0 degrees celsius wet-bulb, 36.8 degrees celsius black-globe, 29.5 degrees--WBGT which are likely to cause heat-related illnesses. 2) The physical load of baseball umpires during the game showed a 1.69 percent decrease in average body weight due to perspiration, a 0.43 degrees C increase in oral temperature and an increase in heart rate. An examination of the serum biochemical elements showed that muscle deviation enzymes changed due to muscular activity and blood condensed due to perspiration. The physical load levels of baseball umpires were influenced by extreme heat and physical activity during the game. 3) There were no observable differences in either the amount of physical activity or the extreme heat environment among the umpires of different field positions. But the chief umpire's physical load showed a greater decrease in body weight, more blood condensation due to perspiration as a result of the heavier equipment he wore, more muscular activity and higher energy consumption than his counterparts on the bases. 4) The umpire's heart rates were higher during games than before games. The moment they were on the playing field. Their heart rates rose to an average of 134. It remained above 115 for about two hours, apparently caused by physical activity and heart load.