A levelling of oxygen uptake (VO2 plateau) at high exercise intensities is conventionally used as the criterion for establishing VO2max during progressive, incremental exercise testing. Only a minority of children, however, demonstrate a VO2 plateau during exercise to voluntary exhaustion. This study was therefore designed to investigate whether a VO2 plateau is required before peak VO2 can be considered a maximal index of children's aerobic fitness. Eighteen girls and 17 boys (age 9.9 +/- 0.4 yrs) carried out three treadmill tests to exhaustion one week apart. The first test comprised a discontinuous, incremental protocol to voluntary exhaustion. In test two each child warmed up and then ran to exhaustion at the same belt speed but on a gradient 2.5% greater than that which had produced an exhaustive effort on the first test. The third test was conducted similarly but the treadmill gradient was raised to 5% greater than that which had produced an exhaustive effort on the first test. Seven girls and 6 boys demonstrated a VO2 plateau (< or = 2 ml.kg-1.min-7) on the first test but no significant differences in either anthropometrical or peak physiological data were detected between those who demonstrated a plateau and those who did not. Mean peak VO2 values during tests two and three (supramaximal tests) did not increase significantly above that achieved on test one although indicators of an increased anaerobic contribution were significantly higher in both supramaximal tests. These findings indicate that peak VO2 in test one was a maximal value despite the absence of a VO2 plateau. The requirement of a VO2 plateau before peak VO2 can be regarded as a maximal index of young children's aerobic fitness is therefore untenable.