Abstract Elevated oxygen uptake (VO2) during moderate-intensity running following a bout of interval running training has been studied previously. To further investigate this phenomenon, the VO2 response to high-intensity exercise was examined following a bout of interval running. Well-trained endurance runners were split into an experimental group [maximum oxygen uptake, VO2max 4.73 (0.39) lmin1] and a reliability group [VO2max 4.77 (0.26) lmin1]. The experimental group completed a training session (4800m at 1kmh1 below speed at VO2max, with 3min rest between each 800-m interval). Five minutes prior to, and 1h following the training session, subjects completed 6min 30s of constant speed, high-intensity running designed to elicit 40% �� (where �� is the difference between VO2 at ventilatory threshold and VO2max; tests 1 and 2, respectively). The slow component of VO2 kinetics was quantified as the difference between the VO2 at 6min and the VO2 at 3min of exercise, i.e. ��VO2(63). The ��VO2(63) was the same in two identical conditions in the reliability group [mean (SD): 0.30 (0.10) lmin1 vs 0.32 (0.13) lmin1]. In the experimental group, the magnitude of the slow component of VO2 kinetics was increased in test 2 compared with test 1 by 24.9% [0.27 (0.14) lmin1 vs 0.34 (0.08)lmin1, P0.05]. The increase in ��VO2(63) in the experimental group was observed in the absence of any significant change in body mass, core temperature or blood lactate concentration, either at the start or end of tests 1 or 2. It is concluded that similar mechanisms may be responsible for the slow component of VO2 kinetics and for the fatigue following the training session. It has been suggested previously that this mechanism may be linked primarily to changes within the active limb, with the recruitment of alternative and/or additional less efficient fibres.