To investigate the physical activity patterns of British primary schoolchildren (mean (SD) 10.7 (0.3) years) the minute by minute heart rates of 67 boys and 65 girls were monitored continuously for three 12 hour periods during normal schooldays. In addition 39 children had their heart rates monitored during a 12 hour period on a Saturday. Few children experienced the volume (frequency, intensity, and duration) of physical activity associated with an improvement in cardiopulmonary fitness. Shorter (five minute) periods of the required intensity were, however, quite common. No difference between moderate amounts of activity was detected between boys and girls, but the boys had more five minute sessions of intense activity than the girls. These findings suggest that more research into the effects of short periods of intense physical activity on the cardiopulmonary systems of young children is required and that the determinants of habitual physical activity require further investigation especially in the context of sex differences at such an early age.