Cross-sectional studies have compared the oxygen uptake (VO2) kinetics during the on-transient of moderate intensity exercise in older and younger adults. The slower values in the older adults may have been due to an age-related reduction in the capacity for O2 transport or alternatively a reduced intramuscular oxidative capacity. We studied: (1) the effects of ageing on VO2 kinetics in older adults on two occasions 9 years apart, and (2) the effect of hyperoxia on VO2 kinetics at the second test time. After a 9 year period, follow-up testing was undertaken on seven older adults (78 +/- 5 years, mean +/- S.D.). They each performed six repeats of 6 min bouts of constant-load cycle exercise from loadless cycling to 80% of their ventilatory threshold. They breathed one of two gas mixtures (euoxia: inspired O2 fraction, FI,O2, 0.21; hyperoxia: FI,O2, 0.70) on different trials determined on a random basis. Breath-by-breath VO2 data were time aligned and ensemble averaged. VO2 kinetics, modelled with a single exponential from phase 2 onset (+20 s) to steady state and described by the exponential time constant (tau) were compared with data collected from the same adults 9 years earlier. One-way repeated measures analysis of variance revealed that tau was slowed significantly with age (from 30 +/- 8 to 46 +/- 10 s), but was unaffected by hyperoxia (43 +/- 15 s). We concluded that: (1) in older adults studied longitudinally over a 9 year period, the on-transient VO2 kinetics are slowed, in agreement with, but to a greater extent, than from cross-sectional data; and (2) the phase 2 time constant (tau) for these older adults was not accelerated by hyperoxic breathing. Thus the expected hyperoxia-induced increase in the capacity for O2 transport was not associated with faster on-transient VO2 kinetics suggesting either that O2 transport may not limit VO2 kinetics during the 8th decade, or that O2 transport was not improved with hyperoxia.