It has been suggested that, during heavy-intensity exercise, O2 delivery may limit oxygen uptake (V̇o2) kinetics; however, there are limited data regarding the relationship of blood flow and V̇o2 kinetics for heavy-intensity exercise. The purpose was to determine the exercise on-transient time course of femoral artery blood flow (Q̇leg) in relation to V̇o2 during heavy-intensity, single-leg, knee-extension exercise. Five young subjects performed five to eight repeats of heavy-intensity exercise with measures of breath-by-breath pulmonary V̇o2 and Doppler ultrasound femoral artery mean blood velocity and vessel diameter. The phase 2 time frame for V̇o2 and Q̇leg was isolated and fit with a monoexponent to characterize the amplitude and time course of the responses. Amplitude of the phase 3 response was also determined. The phase 2 time constant for V̇o2 of 29.0 s and time constant for Q̇leg of 24.5 s were not different. The change (Δ) in V̇o2 response to the end of phase 2 of 0.317 l/min was accompanied by a ΔQ̇leg of 2.35 l/min, giving a ΔQ̇leg-to-ΔV̇o2 ratio of 7.4. A slow-component V̇o2 of 0.098 l/min was accompanied by a further Q̇leg increase of 0.72 l/min (ΔQ̇leg-to-ΔV̇o2 ratio = 7.3). Thus the time course of Q̇leg was similar to that of muscle V̇o2 (as measured by the phase 2 V̇o2 kinetics), and throughout the on-transient the amplitude of the Q̇leg increase achieved (or exceeded) the Q̇leg-to-V̇o2 ratio steady-state relationship (ratio ∼4.9). Additionally, the V̇o2 slow component was accompanied by a relatively large rise in Q̇leg, with the increased O2 delivery meeting the increased V̇o2. Thus, in heavy-intensity, single-leg, knee-extension exercise, the amplitude and kinetics of blood flow to the exercising limb appear to be closely linked to the V̇o2 kinetics.