The physiological adjustments to aerobic work (5.6 km/h, up a 9% grade) and to exhausting treadmill work of former champion middle-distance runnerswere determined in 1971, at ages 47–68 yr, 25–43 yr after their competitive careers in track. In the resting state the former athletes as a group are very much like nonathletes of the same ages. Efficiency in the aerobic walk was the same in both groups and did not change with age in either, but the former athletes on the average performed the walk with lessstrain as indicated by lower blood lactates, “ventilatory equivalents,” and heart rates than nonathletes at corresponding ages. Mean VO2max of the runners declined from 71.4 ml/min-kg-1 in youth to 41.8 at a meanage of 56.6 yr, as compared with mean values of 50.6 and 36.5 ml/min-kg-1innonathletes at corresponding ages. VO2max had declined below the averageof nonathletic men in only two of the former runners. Mean maximal heart rate declined with age from 186 to 180 in the runners, and from 199 to 186 innonathletes at corresponding ages. Ventilatory responses of men in both groupswere closely related to the increases of blood lactate in both aerobic and maximal work.