The present study examined the relationship between peak V̇O2 and habitual physical activity in 11- to 16-year-old students. The peak V̇O2 of 111 girls and 85 boys was determined using treadmill or cycle ergometry. Habitual physical activity was estimated from minute-by-minute heart rate monitoring over three 12-hr periods during normal school days. Over half of the girls and one third of the boys failed to sustain a single 10-min period with their heart rate at or above 140 bpm. Only one boy sustained a daily 20-minute period with a heart rate at or above 160 bpm. During Saturday monitoring over 90% of the girls and 75% of the boys failed to sustain a single 10-min period with their heart rate at or above 140 bpm, and only one girl and four boys sustained a 20-min period with their heart rate at or above 160 bpm. No significant relationship was detected between peak V̇O2 and heart rate indicators of habitual physical activity. This study suggests that few children have periods of physical activity of sufficient intensity and duration to stress the cardiopulmonary system.