Purpose: To examine the time-course of changes in the oxygen uptake (VO₂) kinetics response subsequent to short-term exercise training (i.e., 24h, 48h, 72h, and 120h post-training) and examine the relationship with the time-course of changes in microvascular (deoxygenated hemoglobin concentration ([HHb])/VO₂ ratio) and macrovascular (flow-mediated dilation (FMD)) O₂ delivery to the active tissues/limbs. Methods: Seven healthy older (OA: 74±6yr) and young men (YA: 25±3yr) completed three endurance cycling exercise training sessions at 70% VO_2max. Moderate-intensity exercise on-transient VO2 (measured breath-by-breath) and [HHb] (measured by near-infrared spectroscopy) were modeled with a mono-exponential and normalized (0-100% of response) and the [HHb]/VO₂ ratio was calculated. Ultrasound-derived FMD of the popliteal artery was assessed following 5 minutes of cuff occlusion. %FMD was calculated as the greatest percent change in diameter from baseline. Results: Time constant of VO₂ (VO₂) was significantly reduced in both OA (~18%) and YA (~23%) 24h (P<0.001) post-training and remained decreased at 48h before returning toward PRE values. Both groups showed a significant decrease in the [HHb]/VO₂ ratio at 24h, 48h and 72h (P=0.001, P=0.01 and P=0.03, respectively) post-training before returning toward PRE values at 120h. %FMD followed a similar time course as changes in the [HHb]/VO2 ratio; being significantly greater in both OA (by ~64%) and YA (by ~26%) at 24 h (P<0.001), remaining increased at 48h and 72h (P=0.02 and P=0.03, respectively) and returning toward PRE values at 120h. Conclusion: These data suggest the rate of adjustment of VO₂ may be constrained by O₂ availability in the active tissues.