It has been suggested that during heavy-intensity exercise O₂ delivery may limit VO₂ kinetics, however, there are limited data regarding the relationship of blood flow and VO₂ kinetics for heavy-intensity exercise. The purpose was to determine the exercise on-transient time course of femoral artery blood flow (Qleg) in relation to VO₂ during heavy-intensity single-leg, knee-extension exercise. Five young subjects performed 5-8 repeats of heavy-intensity exercise with measures of breath-by-breath pulmonary VO₂ and Doppler ultrasound femoral artery mean blood velocity and vessel diameter. The phase 2 time frame for VO₂ and Qleg were isolated and fit with a mono-exponential to characterize the amplitude and time course of the responses. Amplitude of the phase 3 response was also determined. The phase 2 VO₂ of 29.0 s and for Qleg of 24.5 s were not different. The delta VO₂ response to the end of phase 2 of 0.317 l.min^-1 was accompanied by a Qleg increase of 2.35 l.min^-1, giving a Q:VO2 of 7.4. A slow component VO₂ of 0.098 l.min^-1 was accompanied by a further Qleg increase of 0.72 l.min^-1 (Qleg:VO2 = 7.3). Thus, the time course of Qleg was similar to that of muscle VO₂ (as measured by the phase 2 VO₂ kinetics), and throughout the on-transient the amplitude of the Qleg increase achieved (or exceeded) the Qleg:VO2 steady state relationship (ratio ~4.9). Additionally, the VO₂ slow component was accompanied by a relatively large rise in Qleg with the increased O₂ delivery meeting the increased VO₂. 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 VO₂ kinetics.