Mineral metabolism and its hormonal regulation will be studied in pregnant laboratory rats and their fetuses. The serum concentration of calcium and phosphorus in the mother and the fetus will be determined along with the circulating levels of the active metabolites of vitamin D, parathyroid hormone and calcitonin. Experiments designed to study the formation of metabolites from 25OHD3 in the mother and the fetus will be conducted in animals rendered D-deficient by dietary restriction or with normal stores of vitamin D. Preliminary results showed that 24, 25-dehydroxyvitamin D3 was the dominant metabolite formed in the fetus while 1,25-dihydroxy-vitamin D3 was the dominant metabolite in the mother. Alterations in the maternal serum levels of 24,25-dihydroxy-vitamin D3 did not change the serum levels of this metabolite in the fetus. The relationship between the maternal and fetal metabolic pathways as well as the role of the placenta in this relationship will be studied. Selected tissues from either the mother or the fetus will be incubated under defined conditions in order to determine the presence or absence of enzymatic pathways for the hydroxylation of 25OHD3 in either the 1 or 24 carbon positions. The factors, chemical and hormonal, which regulate these enzymatic stems will be identified. Those hormones associated with pregnancy, such as the estrogens and placental lactogen, will be tested for their role as regulators of these pathways. Once the maternal and fetal relationship has been clarified, the onset of ossification of the fetal skeleton and the hormonal changes surrounding this unique event in mineral metabolism will be defined.