The effectiveness of beverage composition will be evaluated by modifying the sodium content and carbohydrate content of the beverage. Hypothesis: Completeness of rehydration will be greater in the beverage with higher sodium concentration. It is also hypothesized that electrolyte-carbohydrate beverages will promote rehydration that exceeds that of consuming placebo (water). Using a randomized balanced-treatment crossover design, physically trained males (18-35 yrs) will participate in intermittent exercise for 85 to 100 minutes in three experimental trials of variable intensity training in the heat (30 +/-3 C) followed by a rehydration-recovery period. Exercise will be done at varying intensities (intermittent intervals) ranging from 50% to nearly 100% of maximum oxygen uptake (max VO2) determined prior to the experimental trials. During the 85 to 100 min of exercise, no fluids will be given during the trial to elicit an acute reduction in body mass of approximately 2.5 to 3% (standardized for each subject). Following exercise, 100% of the volume of fluid lost will be replaced with one of the beverages. Beverages will be ingested in six aliquots over a 1-hr period given at the end of the trial. Retention of fluid will be calculated by measuring volume of urine lost at 30, 60, 135, and 210 min following ingestion of the rehydration beverage. The primary outcome variable is the percentage of fluid retained, i.e., the difference between volume ingested and cumulative urine produced in 3.5 h as a percentage of volume ingested. Body mass, urine specific gravity, total body water, and ratings of vigor and affect will be assessed prior to exercise, after exercise in a state of dehydration, and at 210 min post-exercise following rehydration. A one-way ANOVA will be used to test the hypothesis for rehydration, the primary outcome variable.