1. Eighty apparently healthy sedentary males, aged 31–69 years, undertook a multistage treadmill walking test that was terminated at maximum tolerable effort. 2. An electrocardiogram (ECG), blood pressures and respiratory metabolism measures were taken at rest, during the walk and for 15 min post-exercise. The mean values for four age groups of twenty subjects each were analysed statistically. 3. A near rectilinear decrease in walk-time and maximum oxygen uptake (V̇o2max.) (ml min−1 kg−1) with advancing age was observed. A difference in V̇o2max. of 26·5% between the oldest and youngest groups was decreased to 17·7% when expressed in ml min−1 kg−1 of lean body weight (LBW). Heart rate (HR) and pulmonary ventilation (V̇e) at maximum tolerable effort also declined with age. 4. There were no age-related differences in V̇e and HR during submaximal work, whereas significant differences in V̇o2 were observed only during early stages. 5. It was concluded that the decline in maximum performance with age was not due to differences in the efficiency of aerobic energy utilization but to factors limiting energy production. 6. Prediction of walk-time utilizing anthropometric, resting and submaximal work, measurements proved unsatisfactory. Although an r value of −0·69 between age and walk-time was obtained, V̇o2max. and walk-time yielded an r or 0·93, thus indicating that chronological age alone was not adequate to assess work capacity.