The purpose of this study was to examine the effect of endurance training on oxygen uptake (O₂) kinetics during moderate [below the lactate threshold (LT)] and heavy (above LT) treadmill running. Twenty-three healthy physical education students undertook 6 wk of endurance training that involved continuous and interval running training 3-5 days per week for 20-30 min per session. Before and after the training program, the subjects performed an incremental treadmill test to exhaustion for determination of the LT and the O_2 max and a series of 6-min square-wave transitions from rest to running speeds calculated to require 80% of the LT and 50% of the difference between LT and maximal O₂. The training program caused small (3-4%) but significant increases in LT and maximal O₂ (P < 0.05). The O₂ kinetics for moderate exercise were not significantly affected by training. For heavy exercise, the time constant and amplitude of the fast component were not significantly affected by training, but the amplitude of the O₂ slow component was significantly reduced from 321 ± 32 to 217 ± 23 ml/min (P < 0.05). The reduction in the slow component was not significantly correlated to the reduction in blood lactate concentration (r = 0.39). Although the reduction in the slow component was significantly related to the reduction in minute ventilation (r = 0.46; P < 0.05), it was calculated that only 9-14% of the slow component could be attributed to the change in minute ventilation. We conclude that the O₂ slow component during treadmill running can be attenuated with a short-term program of endurance running training.