Abstract. The purpose of the present study was to examine comprehensively the kinetics of oxygen uptake ([MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] ) during treadmill running across the moderate, heavy and severe exercise intensity domains. Nine subjects [mean (SD age, 27 (7)years; mass, 69.8 (9.0)kg; maximum [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] , [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] , 4,137 (697)mlmin1] performed a series of "square-wave" rest-to-exercise transitions of 6min duration at running speeds equivalent to 80% and 100% of the [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] at lactate threshold (LT; moderate exercise); and at 20%, 40%, 60%, 80% and 100% of the difference between the [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] at LT and [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] (, heavy and severe exercise). Critical velocity (CV) was also determined using four maximal treadmill runs designed to result in exhaustion in 215min. The [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] response was modelled using non-linear regression techniques. As expected, the amplitude of the [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] primary component increased with exercise intensity [from 1,868 (136)mlmin1 at 80% LT to 3,296 (218)mlmin1 at 100% , P0.05]. However, there was a non-significant trend for the "gain" of the primary component to decrease as exercise intensity increased [181 (7)mlkg1km1 at 80% LT to 160 (6)mlkg1km1 at 100% ]. The time constant of the primary component was not different between supra-LT running speeds (mean value range = 17.919.1s), but was significantly shorter during the 80% LT trial [12.7 (1.4)s, P0.05]. The [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] slow component increased with exercise intensity from 139 (39)mlmin1 at 20% to 487 (57)mlmin1 at 80% (P0.05), but decreased to 317 (84)mlmin1 during the 100% trial (P0.05). During both the 80% and 100% trials, the [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] at the end of exercise reached [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] [4,152 (242)mlmin1 and 4,154 (114)mlmin1, respectively]. Our results suggest that the "gain" of the primary component is not constant as exercise intensity increases across the moderate, heavy and severe domains of treadmill running. These intensity-dependent changes in the amplitudes and kinetics of the [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] response profiles may be associated with the changing patterns of muscle fibre recruitment that occur as exercise intensity increases.