To study the effects of training and aging on aerobic capacity, 8-wk-old female Wistar rats were assigned to training and sedentary, free-eating control groups (N = 10, each). Training involved 5 wk of daily progressive treadmill running reaching an intensity of 26.8 m.min−1on 15% grade for 1 h. Training was maintained at this intensity for an additional 11 wk. The O2maxof trained and sedentary control rats was assessed every 2 wk. Peak O2max(25.4±3.2 ml-min-1; X → SEM) was observed in the trained group at 20 wk of age and represented a 15% increase over the untrained group. When expressed on a relative basis, the peak Vo2maxin trained rats at 20 wk of age was 86.6±8.6 ml · kg−1.min−1or an 8% improvement compared to controls. In control animals &OV0312;O2maxwas also greatest at 20 wk of age (22.0 ± 1.9 ml-min−1; 79.9±6.4 ml-kg−1-min−1). After 20 wk of age, &OV0312;O2maxdeclined in sedentary controls, but was maintained in trained animals through 24 wk of age. Mature trained female animals were approximately 6% heavier than sedentary free-eating controls. Endurance training may stimulate anabolic processes as well as improve and maintain aerobic capacity in the female rat.