A fundamental change that occurs with advancing age is a diminishing maximal aerobic power (VO2 max). In sedentary individuals the rate of decline in VO2 max as a function of age is estimated at 10% per ten years after age 20 but becomes more gradual in endurance-trained individuals regardless of gender, approaching about 5% with each ten year span. It is thought that reduced sweating and vasomotor responses to heat stress indicate deterioration in thermoregulatory function. A scrutiny of thermoregulatory mechanisms in the aging process is necessary because other quantitative studies of actual efferent thermoregulatory drive point to a maintenance of thermoregulatory competence well into the sixth decade of life provided a level of aerobic fitness is sustained. Pandolf et al studied a group of nine young and nine middle-aged men having parallel body weights, skin surface areas, percent body fat and maximal aerobic power. These groups displayed almost equivalent thermoregulatory responses to comparable exercise and heat stress conditions following 10-days of heat acclimation. The present paper is an examination of the above study's thermoregulatory responses during exercise transients. Transient analysis to specific exercise/ heat stress provocation provides a clearer understanding of the influence of age and aerobic fitness on thermoregulatory control of sweating than steady-state analysis. Keywords: Ageing; Body temperature control; Exercise physiology.