Purpose: To compare the physiological and perceptual responses during treadmill walking at a self-regulated intensity in women between 18-25 years and 40-55 years. Methods: Ten young (18-25 y) women (YW) and eleven middle-aged (40-55 y) women(MW) participated in the study. Participants walked on a treadmill at a self-selected intensity until they expended 200 kcal. Expired gases and HR were continuously collected using open circuit spirometry and telemetry respectively. RPE was recorded, and blood samples were drawn at 5 min intervals and during the final min of exercise. Results: The time required to expend 200 kcal during self-regulated treadmill exercise was significantly longer in MW than YW (31.4± 2.9 min vs. 27.1 ± 3.3 min). The treadmill velocity was significantly higher and the %VO2 max was significantly lower in YW than MW women. Overall and differentiated rating ratings of perceived exertion for the legs and chest were similar in YW and MW, and were within the range of light to somewhat hard. Conclusion: When allowed to self-select their exercise intensity both YW and MW select an intensity within the range considered safe and effective for the development and maintenance of health and cardiorespiratory fitness.