The primary research objective is to assess capacity for exercise in desert heat for Caucasians, Blacks, and Chicanos as related to age, sex, and body fat. Based on successful past experience, a class of 50 joggers (both sexes and a wide age range) will be recruited each semester: Caucasians for the first year, Black the second year and Chicanos the third year. During an eight-week jogging course, those with satisfactory stress electrocardiograms will be invited to volunteer for laboratory studies designed to yield a physiological profile: pulmonary functions, blood pressure, body fat, aerobic capacity and steady state performance on the treadmill, at about25, 50 and 75% aerobic capacity. Each summer fifty will be invited to come to nearby Boulder City for three walks in desert heat, temperatures 37 to 42 C. Caucasians will be studied the first year, Blacks the second and Chicanos the third year. Physiological assessment will include metabolic and cardiovascular responses and temperature regulation. Laboratory and desert performances will yield an excellent measure of physiological age. The difference between physiological and chronological ages for each sex and three races will be related to life habits such as exercise, cigarette smoking and caloric intake, adequate or excessive.