Age-related changes in the number of cartilage cells within the proximal growth plate of the humerus were determined in normal ICR male mice. The rates of reduction in the number of proliferative and hypertrophic chondrocytes were calculated and compared during the animals’ first 6 months of life. It was found that during the period of highest growth activity, between weanling and sexual maturation, the rate of cell production is hardly decreased, thus maintaining the thickness of the cartilaginous growth plate and thereby enabling an extensive bone growth. A close correlation was found between the following skeletal activities: decrease in longitudinal bone growth, narrowing of the epiphyseal growth plate and a decrease in the proliferative activity of young chondrocyte