The pattern of body weight gain, the longitudinal growth of the humerus and the changes in the width of its proximal growth plate were determined in normal ICR male mice. The respective growth rates were calculated and compared throughout the animals’ first year of life. It was found that the overall growth period can be divided into four phases which differ markedly in their growth rates. Whereas skeletal maturation is reached at the age of 20 weeks, the animals’ body weight continues to increase up to the end of the 52nd week. The humeral growth plate retains its existence and function until old age is reached, i.e. 15 months. During the period of highest growth activity extending between weanling and sexual maturation, the growth velocities of the various parameters do not coinc