In this study, we investigated total sweat rate per unit area of body surface per hour (TSR), the percentage of total sweat loss to initial body weight (%TSL) and the percentage of fluid intake volume to total sweat loss (%FIV) in children (during baseball or basketball practice), young adults (during softball or baseball practice), and the elderly (during gateball practice) in summer. During the practice all participants were given access to drink sports drink or Japanese tea ad libitum. In the basketball group of children no sex difference was observed in TSR, %TSL and %FIV. Although TSR and %TSL in the baseball group of children were significantly higher than in the basketball group due to a higher WBGT, there was no group difference for %FIV. Children drank sports drinks (with carbohydrate-electrolyte) of the similar volume with total sweat loss. The baseball group of children showed a significantly greater %FIV compared to the young adult group regardless of simi lar %TSL, when they drank sports drinks. Children showed a significantly higher %FIV in sports drinks than in Japanese tea, whereas the %FIV of young adults was not affected by the type of beverage. The TSR, %TSL and %FIV of the elderly group were significantly lower than in young adults and children. These results suggest that children can prevent voluntary dehydration by con-suming sports drinks with carbohydrate-electrolyte ad libitum regardless of sex and WBGT, and that periodic drinking should be actively encouraged for the elderly because of their blunted rehydration ability, to prevent heat disorders during sports activity in summer.