The purpose of this study was to examine the effect of umpires' intake of water during a summertime baseball game, on serum and urinary biochemical elements, body weight and physiological factors. Twenty-eight umpires were classified into two groups. Group W was composed of 14 umpires who had water while those who did not were group C. Group W had 250 ml of water that was cooled to about 5 degrees C after the 5th inning of the game. The game was played in the following environment: atmosphere temperature was 31 degrees C; wet-bulb temperature was 26 degrees C; blackglove temperature was 40 degrees C; the velocity of the wind was 1.18 m/sec in bright sunshine on a hot and moist day. The results obtained were as follows. In group W, more body weight was lost than in group C, but statistically there was no significant difference. Urinary volume in group W became smaller than in group C, but statistically there was no significant difference. Water in blood increased in group W and decreased in group C but statistically there was significant difference. In both groups, oral temperature decreased, but only in group C was there a statistically significant difference. Although the intake of water increased the volume of perspiration, it did not dilute the concentrated blood, suppress the rise of body temperature, or result in a rise of low blood pressure.