This is a report on research in the field of physical responses of children to strenuous activity. The paper is divided into three subtopics: (1) peak performance measure in children; (2) training effects on children; and (3) importance of physical activity for children. Measurements used are oxygen consumption, ventilation, heart rate, cardiac output, and work rate. Children between the ages of 6 and 13 are the subjects. Conclusions indicate that strenuous physical activity has beneficial effects in the physiological development of the child, which apparently carries over into adulthood and that physical activity is needed for normal growth and development. However, consideration should be given to the possibility of stress fractures and related bone disorders that may occur in children especially during a growth spurt. Suggestions are given on the best ways to provide healthful exercise for children. (JD)