Although morbidity and mortality data consistently demonstrate the older individuals are at greater risk in hot environments, few investigators have studied the relationship between age and thermoregulatory response. It is unclear to what extent decreased heat tolerance in the elderly is a function of decreased aerobic capacity, as opposed to age-related changes in neural function, functional capacity of the sweat gland, etc. If decreased thermoregulatory function is linked solely with lowered fitness, physical conditioning of older individuals could potentially result in improved heat tolerance. Previous investigations in this area have suffered from the inability to match groups of older and younger subjects with respect to aerobic capacity. We have alleviated that problem by recruiting a subject population of highly fit postmenopausal women over the age of 55 which we propose to compare with (1) younger women of similar aerobic fitness, (2) postmenopausal women with lower fitness levels, and (3) very fit younger women. This investigation proposes to systematically compare physiological effector (body temperature, cardiovascular, and sudomotor) responses to hyperthermia among these groups. Subjects will be tested at rest and during moderate exercise in 3 environments (neutral, hot-dry, and warm-humid) before and after acclimation. The specific aims of this research include the following comparisons among the aforementioned groups. 1. onset characteristics (temporal and temperature thresholds) of effector responses (sweating and cutaneous vasodilation); 2. body temperature, heart rate, blood pressure, and sweating responses to moderate exercise in the 3 environments; 3. differences in heat-activated sweat gland number and density, and sweat production per gland; 4. percentage changes in blood volume, red cell volume, and plasma volume due to exercise in these environments, and 5. alterations in magnitude and control of cutaneous blood flow and cardiac output. In summary, we propose to examine the extent to which differences in aerobic capacity conceivably contribute to age related decrements in thermal tolerance.