The purpose of this study was to examine 12-h heart rate patterns during the summer using a holter monitoring system (Oxford Instruments) in prepubescent children. Parental consent was obtained for the 40 volunteers (22 boys, 18 girls) ages 6 to 7 years. Heart rates exceeding 160 bts min-1 occurred 20.9 min and 9.4 min for the boys and girls, respectively, during the 12-h monitoring period (8:00 a.m.-8:00 p.m.). When comparing the heart rate profiles between the sexes, the boys had significantly (P less than 0.05) more heart beats at a level of 140 bts min-1 and above than the girls. Even though the children appear moderately active, they very seldom experience high intensity physical activity. Furthermore, the heart rate patterns show that the boys are more physically active than the girls.