The purpose of this investigation was to determine the contribution of muscle O2 consumption (m O2) to pulmonary O2 uptake (p O2) during both low-intensity (LI) and high-intensity (HI) knee extension exercise, and during subsequent recovery, in humans. Seven healthy male subjects (age 20-25 years) completed a series of LI and HI square-wave exercise tests in which m O2 (direct Fick technique) and p O2 (indirect calorimetry) were measured simultaneously. The mean blood transit time (MTT) from the muscle capillaries to the lung was also estimated (based on measured blood transit times from femoral artery to vein and vein to artery). The kinetics of m O2 and p O2 were modelled using non-linear regression. The time constant (tau) describing the phase II p O2 kinetics following the onset of exercise was not significantly different from the mean response time (initial time delay + &tgr) for m O2 kinetics for LI (30 +/- 3 vs. 30 +/- 3 s) but was slightly higher (P<0.05) for HI (32 +/- 3 s vs. 29 +/- 4); the responses were closely correlated (r = 0.95 and r = 0.95; P<0.01) for both intensities. In recovery, agreement between the responses was more limited both for LI (36 +/- 4 vs. 18 +/- 4 s, P<0.05; r = -0.01) and HI (33 +/- 3 vs. 27 +/- 3, P>0.05; r = -0.04). MTT was ~17 s just before exercise and decreased to 10 s and 12 s after 5 s of exercise for LI and HI, respectively. These data indicate that the phase II p O2 kinetics reflect m O2 kinetics during exercise but not during recovery where caution in data interpretation is advised. Increased m O2 likely makes a small contribution to p O2 during the first 15-20 s of exercise.