The rate of adjustment of oxidative phosphorylation has been shown to be slower in older compared to young adults and in untrained compared to endurance-trained individuals. However, it remains unclear to what extent chronic endurance training can speed oxygen uptake (VO2) kinetics in groups of young, middle-aged, and older adults. We examined a group of young, middle-age, and older chronically trained and untrained males to compare the rate of adjustment of oxidative phosphorylation in each age-group and condition. Pulmonary VO2 (VO2p) was measured during repeated step transitions from 20 W to moderate-intensity cycling (80-90% of estimated lactate threshold). VO2p was measured breath-by-breath using a volume turbine and a mass spectrometer. VO2p profiles were modeled as a mono-exponential using non-linear regression. The groups consisted of young (Y) trained (n= 4) and untrained (n= 9) (24 ± 5 yrs.; mean ± SD), middle-aged (M) trained (n= 5) and untrained (n= 4)) (51 ± 3 yrs.), and older (O) trained (n= 3) and untrained (n= 6) (68 ± 4 yrs.) males. Maximal VO2 (VO2max) was lower in O (2.55 ± 0.48 L/min) compared to Y (4.12 ± 0.51 L/min) and M (4.00 ± 0.50 L/min) (p< 0.05) and it was larger in trained (4.04 ± 0.74 L/min) compared to untrained (3.37 ± 0.84 L/min) individuals. The rate of adjustment of oxidative phosphorylation, as represented by the phase 2 VO2 time-constant (τVO2p), was longer in O (39.1 ± 13.3 s) compared to Y (22.2 ± 8.7 s) and M (24.1 ± 7.3 s) (p< 0.05) and it was shorter in trained (19.4 ± 7.4 s) compared with untrained (32.9 ± 11.8 s) individuals (p< 0.05). There was a significant age-group by condition interaction indicating that O untrained (46.8 ± 8.1 s) but not trained (23.8 ± 4.2 s) had a slower VO2 kinetics compared to the other groups (p< 0.05). These preliminary data support the idea that: 1) Chronic endurance trained results in faster rate of adjustment of oxidative phosphorylation; 2) the age-related slowness of VO2 kinetics is only observed in older but not middle-aged males; 3) chronic endurance training results in complete abolishment of any age-related lengthen of the phase 2 VO2p.