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This investigation evaluated the new National Collegiate Athletic Association model of heat acclimatization for football players using physiological, psychological, fluid balance, anthropometric, and nutritional variables. Eleven football players (20 [+ or -] 1 year, 1.88 [+ or -] 0.05 m, and 115.36 [+ or -] 18.85 kg) from a Division I football team were observed for the first 8 days of preseason practices. Measurements such as heart rate and gastrointestinal temperature ([T.sub.GI]) via telemetric sensor were taken before, 3 times during, and after practice daily. An average 1.39-kg (1.2%) decrease of body mass occurred from prepractice to postpractice (p < 0.01). Consistent with mild body mass losses, urinary indices of hydration status (i.e., color, specific gravity, and osmolality) indicated mild fluid deficits. A significant increase (p < 0.05) from pre- to postpractice was observed in urine color and urine specific gravity, but chronic hypohydration over the 8 days was not noted. The Environmental Symptoms Questionnaire (ESQ) postpractice score was significantly higher (p < 0.05) than the prepractice score was, but averages did not differ across practice days. There was no difference in postpractice [T.sub.GI] measurements across days (p < 0.05). Heart rate, [T.sub.GI], and ESQ measurements indicated that football players experienced gradual heat acclimatization and enhanced heat tolerance, despite progressive increases of exercise variables, clothing, and environmental stressors. Key Words. core body temperature, hydration, football uniform, surface area-to-mass ratio