The experiments reported in this paper were designed to gain information on the growth and development of the bone mass of the rat and to recognize genetically determined features of the growth phenomenon. The experimental model involved the study of the axial (AX, head excluded) and appendicular (AP) skeletons of two strains of inbred rats ('m' and 'e') from the 3rd to the 27th week of age. The contribution of the skeleton to body weight differed between strains ('m' greater than 'e'). The development of the skeleton was in phase with body growth (peak growth rate = 7th week of age). The AP skeleton of these strains of rats differed in the kinetics of matrix mineralization ('m' faster than 'e') and in the ash/matrix ratio of adult animals ('m' greater than 'e'). The AX skeleton, on the other hand, showed a constant ash/matrix ratio from the 4th to the 27th week of age, without significant differences between strains. The anatomical volume of the femur was found to differ significantly between strains at an earlier age (4th week) than its dry weight (7th week). The AP and AX skeleton of 'e' rats mature simultaneously attaining their peak calcium mass at 36 weeks of age. The maturation of the AP and AX skeletons of 'm' rats, on the other hand, are dissociated and attain full maturity at approximately 39 and 57 weeks, respectively. The curves of specific growth rates (g of Ca (or matrix) per week/g of skeletal Ca (or matrix] of the AP and AX skeletons indicate that there exist separate metabolic controls for the growth of Ca and matrix masses. Absolute measurements (Ca, organic matrix, ash or total dry weights, bone volume) unambiguously established that 'm' rats have greater bone mass than 'e' ones. Relative measurements favoured 'm' (ash/matrix ratio of AP skeleton), 'e' (femoral weight/volume ratio, percentage of metaphyseal trabecular bone volume) or gave no significant differences between strains (ash/matrix ratio of AX skeleton). These results indicate that the AP and AX skeletons have important metabolic differences and that their growth and development are under genetic control.