The purpose of this study was to investigate the effect of exercise mode on the characteristics of the oxygen uptake (VO(2)) ()response to exercise within the severe intensity domain. Twelve participants each performed a treadmill running test and a cycle ergometer test to fatigue at intensities selected to elicit a mode-specific VO(2)max and to cause fatigue in ~5 min. The tests were at 234 (30) m.min(-1) and 251 (59) W, and times to fatigue were 297 (15) s and 298 (14) s, respectively. The overall rapidity of the VO(2)response was influenced by exercise mode [VO(2)max was achieved after 115 (20) s in running versus 207 (36) s in cycling; p<0.01]. VO(2) responses were fit to a three-phase exponential model. The time constant of the primary phase was faster in treadmill tests than in cycle ergometer tests [14 (6) s versus 25 (4) s; p<0.01], and the amplitude of the primary phase was greater in running than in cycling when it was expressed in absolute terms [2327 (393) ml.min(-1) versus 2036 (301) ml.min(-1); p=0.02] but not when it was expressed as a percentage of the total increase in VO(2) [86 (6)% versus 82 (6)%; p=0.09]. When quantified as the difference between the end-exercise VO(2) and the VO(2) at 2 min, the amplitude of the slow component was ~40% smaller in running [177 (92) ml.min(-1) versus 299 (153) ml min(-1); p=0.03]. It is concluded that exercise modality affects the characteristics of the VO(2) response at equivalent intensities in the severe domain.