Recent interest in Master's track competition led to an investigation of 25 champion American runners from 40 to 75 years of age. In the past year, all the subjects had performed and placed in either a regional or national meet. The results showed all groups to have excellent cardiovascular and body composition characteristics which were particularly apparent when the groups were compared to sedentary and moderately trained subjects of similar ages. Although maximal performance and oxygen intake decreased with age, a dramatic reduction did not occur until after age 60. Body composition measures, maximal pulmonary ventilation, resting heart rate and blood pressure, and serum lipids were similar for subjects up to age 65, but showed a significant decrement thereafter. The causes of the decrement in performance and physiological function with age were determined to result from a combination of factors, e.g., aging, quantity and quality of conditioning, and number of years in training.