The early osteoporosis in OXYS rats is the presentation of accelerated senescence and earlier positioned as senile. The present study shows the changes in metabolism detected in OXYS rats in the postnatal period. They lead to the development of osteoporosis in future and may underlie the formation of reduced peak bone mass. 90 males OXYS rats used in this study aged from 10 days to 24 months and the control group consisted of 90 male Wistar rats of the matched ages. No differences in BMD in OXYS and Wistar rats at the age of 10 days and 3 months was revealed. At the age of 10 days the OXYS rats showed the higher by 40% activity of ALH--the marker of osteoblast activity--than Wistar rats; but at the age of 3 months ALH activity in OXYS was lower than in Wistar rats. The peak bone mass and BMD in Wistar rats is formed by the age of 12 months, in OXYS rats already by 6, but it did not reach the level of Wistar. The content of Ca in the blood and bone tissue changes similarly: no difference in young age, but reduces in OXYS rats after 6 months to the background of enhanced Ca excretion in urine. However, changing the mineral composition of bone in OXYS rats did not affect the mechanical strength: the absolute strength of the long bones in OXYS at 12 months was lower than that of Wistar, but at the expense of decrease by 1,7 times the cross-sectional area. We suppose that genetically determined hypoplasia of the bone tissue in OXYS rats is the starts of pathogenetic mechanisms of idiopathic osteoporosis.