Five older men (aged 60-69 yr) and five young men (aged 21-29 yr) with approximately equal levels of age-corrected VO2 max were compared with respect to oxygen kinetics at equal absolute workloads (100 watts) and at equal relative workloads (45% VO2 max) on a cycle ergometer. At 45% VO2 max, half times for VO2 response to instantaneous transition from unloaded pedalling were 30.0 s and 27.4 s for old and young respectively (t = 0.260, p less than 0.80). No significant differences were found in the VE response and by inference none existed in O2 extraction. Mean half times for heart rate responses at a workload of 100 W were 24.2 s and 20.6 s for old and young groups respectively (t = 0.722, p less than 0.49). Mechanical efficiency estimated from steady state data at 100 W was 19.8% and 20.5% for old and young groups respectively (t = 0.574). The close similarity in responses to submaximal work in old and young subjects of equivalent fitness suggests caution in the interpretation of agewise decrements observed in physiological variables which may be sensitive to physical fitness status.