Exercise is generally held to be a significant factor in the growth, development, and health of children and adolescents. The effects of physical activity regimens on general growth, as well as quantitative and qualitative changes, in animal muscle and bone tissue have been clearly demonstrated. Less is known about the role of exercise and related metabolic factors in influencing morphological and functional changes in children. The significance of physical activity in controlling body weight and maintaining a healthy balance among the tissue components of growing children has been repeatedly demonstrated. Similarly, research has shown the positive effects of vigorous exercise regimens on muscular strength, muscular endurance, and physical working capacity of children. Recent research points to a complex interaction of neural, hormonal, and metabolic factors in response to the stresses imposed by exercise--the effect on tissue and organ growth being a reflection of the nature, duration, and intensity of the exercise regimen as well as the maturational level and exercise tolerance of children. That certain, as yet underfined, levels of physical activity are essential to the normal growth and health of children would seem to be self-evident. Based on present knowledge, these levels will vary with the nature of the physical activity program and the functional capacity and maturational level of the child. (Author/PB)