The rate of fixation of Ca in the skeleton, striated muscle and liver of a rep-tile, Natrix natrix, was studied in 14 animals aged 2-17 years, using Ca Contrary to findings in rats one observes in these reptiles a quasi-linear decrease of the rate of fixation of Ca45 in the vertebrae, and this phenomenon can be traced up to a relatively old age. On the contrary this rate remains constant between 2 and 17 years in the muscle and between 6 and 17 years in the liver.