The purpose of this thesis was to assess the adaptation of middle-aged and older females to a three to six month endurance training program. Sixty-nine subjects were divided into four age groups to determine the effect of aging on cardiorespiratory improvements. Two treadmill tests for maximal aerobic power were administered prior to training and following the subjects' ability to run a continuous two miles. As a result of training, maximal oxygen consumption (max V02) and maximal ventilation (max VE) increased significantly in all age groups with the youngest group exhibiting the greatest gains and the oldest group showing the least improvements. Age tended to effect max V02 at both T1 and T2 but it also played a role in interfering with the ability to improve in the 55 to 65 age range. Training had little effect on the resting variables observed in this study. Nor did age effect basal parameters such as body weight, serum cholesterol and resting heart rate. However, age did become significant when fat percent and blood pressures were observed.