We compared the kinetics of oxygen uptake (VO2) and phosphocreatine (PCr) during the adjustment to and recovery from plantar flexion exercise in moderately active older (n = 10, 66.9 years) and younger (n = 10, 27.5 years) individuals. VO2 kinetics were similar in the two groups, with time constants (tau) averaging 46.3 +/- 10.2 s (younger, on-transient), 38.1 +/- 14.4 s (younger, off-transient), 46.3 +/- 17.8 (older, on-transient) and 40.7 +/- 19.2 s (older, off-transient). These were similar to corresponding PCr kinetics, measured by 31P nuclear magnetic resonance spectroscopy, which averaged 50.6 +/- 24.0 s (younger, on-transient), 42.0 +/- 16.1 s (younger, off-transient), 39.8 +/- 22.0 s (older, on-transient) and 37.6 +/- 21.6 s (older, off-transient). On-transient tau values for VO2 and PCr were correlated, for combined groups (r = 0.53; P = 0.015). We conclude that: (1) VO2 and PCr kinetics during exercise of a muscle group accustomed to daily activity are not compromised in physically active older humans, and (2) PCr kinetics reflect the kinetics of muscle O2 consumption, and are expressed at the lung (VO2 kinetics) after a transit delay.