The purpose of this study was to examine the thermoregulatory responses and hydration changes of adolescent high school football players during formal preseason football practices under varying environmental conditions. Twenty-five heat-acclimatized male football players (mean±SD: 15±y, Tanner Stage 4±1, 180±8cm, 81.4±15.8kg, 12±5%fat) volunteered to be observed for the initial 10 days of preseason practices. Practices were conducted outdoors in late August. Days 1-5 consisted of one practice; days 6-8, two practices; and days 9-10, one practice. Average maximum WBGT was 23±4°C. Gastrointestinal temperature (T_GI) was measured throughout practice with an ingestible telemetric sensor. Stress state was estimated by measuring the ratio of salivary testosterone and cortisol concentrations (T/C) on days 1, 4, 7, & 9 in the AM (upon the subject waking) and PM (upon the close of practice). Perceptual measures of thirst and thermal sensation and an environmental symptoms questionnaire (ESQ) were collected before, during, and/or after practice. Hydration status was determined AM and PM via urine sample. Sweat rate (SR), forearm sweat composition, and fluid consumed (FC) were measured at selected practices. Results were compared overall and by age groups (Y:14-15y, n=13 and 0:16-17y, n=12). Maximum daily TGI significantly changed across days (F _9,135= 8.336, p<0.001) and correlated strongly with maximum WBGT (r=0.77, p=0.009). Average U_osm indicated subjects were mildly hypohydrated each day at AM and PM. There were no significant U_osm differences in AM measures (p>0.05) or PM measures (p>0.05) across days. Younger players had significantly lower (p=0.02) SR than older players (0.6±0.2 L/hr and 0.8±0.1 L/hr, respectively). SR had a strong and significant correlation with maximum WBGT (r=0.788, p=0.007). Significant differences in AM (p=0.007) and PM (p<0.001) T/C values existed. Younger adolescents had significantly lower T/C values than older adolescents in AM (p=0.025). There were significant differences in post ESQ scores across days(p<0.001). In conclusion, T_GI remained under 40°C possibly due to safe practice guidelines and already established heat-acclimatization. Since subjects had beneficial adaptations in regards to T_GI and sweat variables due to heat-acclimatization, these thermoregulatory mechanisms had the greatest relationship with environmental conditions across all days. Adolescents adequately replaced total sweat loss during practice but remained mildly throughout practice and the week indicating inadequate hydration habits outside of practice. T/C responses may have been due to safe training level stress, environmental conditions, or pubertal changes.