The leveling of oxygen uptake at high work loads has been utilized to define maximal VO2 during progressive exercise testing. Exercise studies in children have indicated, however, that a VO2 plateau can commonly be demonstrated in less than one half of subjects. To examine the potential contributions of subject effort, aerobic fitness, and nonaerobic performance to this variability, 15 children ages 7 to 10 years performed progressive treadmill walking to exhaustion with a ramp protocol. The VO2 plateau was defined as a change in VO2 during the final minute of exercise less than 2 SD below the mean of increases between the previous 4 to 5 submaximal minutes. Five subjects (33.3 percent) demonstrated a plateau. No significant differences in mean peak VO2, heart rate, or respiratory exchange ratio were observed between plateau and nonplateau groups. Testing results of speed (50-yard sprint) and leg power (vertical jump) were also similar. These findings suggest that following: (1) subject effort, aerobic fitness, and nonaerobic factors do not explain the presence or absence of a VO2 plateau during exercise testing of children; and (2) a VO2 plateau should not be used as a requirement for defining VO2max during treadmill testing in this age group.