The purpose of this investigation was to determine the age-related metabolic responses to chronic exercise. Female Fischer 344 rats 3, 12, and 24 mo of age were trained for 8 wks by treadmill running at 75% of their maximal oxygen uptake (VO2max). Values of VO2max were 74.5 +/- 3.1, 63.3 +/- 3.9, and 54.6 +/- 2.3 ml X kg-1 X min-1 in the untrained 3-, 12-, and 24-mo groups, respectively. In response to training, VO2max increased significantly (P less than 0.05) by 11.9, 18.6, and 20.4% in the three groups. The 24-mo animals also demonstrated the greatest improvement in the treadmill speed eliciting VO2max (71%) when compared with the 12-mo (64%) and 3-mo (57%) age groups. While large increases in endurance times were recorded for all trained groups (456, 806, and 324% in 3, 12, and 24 mo, respectively) relative to the controls, the older animals exhibited a diminished response, comparatively. Body weights did not differ between the trained (Tr) and sedentary controls for 3- and 12-mo animals, but the 24-mo Tr rats demonstrated a more rapid decline in body weight with age than did the age-matched controls. While no differences in absolute or relative heart weights existed for the 3- and 12-mo groups with training, both of these parameters in the 24-mo Tr animals were significantly greater than sedentary controls. It was concluded that compared with younger animals the 24-mo group demonstrated a similar relative adaptation to training in terms of aerobic power but a diminished response comparatively in endurance capacity.(ABSTRACT TRUNCATED AT 250 WORDS)