The hormonal mechanisms responsible for growth of the fetus are poorly understood. Most of the anabolic hormones known to stimulate postnatal growth have been shown to exert little or no effect on fetal growth. Only insulin and certain insulin-like peptides have been shown to have effects on fetal growth which might be of physiological importance. The purpose of this project is to test the hypothesis that somatomedin-C is a primary stimulator of fetal growth. The specific objectives are: 1) to study the role of somatomedin-C, insulin, and other anabolic peptides on the stimulation of growth of fetal tissues in organ culture; 2) to determine radioimmunoassayable levels of somatomedin-C in fetal and cord serum and to examine by radioimmunoassay whether growth hormone prolactin, or placental lactogen have the capacity to induce the formation of somatomedin-C in the fetus and young animal; 3) to develop a practical method for quantitating the somatomedin binding protein and to measure this protein serum; 4) to determine whether there is maternal-to-fetal or fetal-to-maternal transfer of somatomedin-C, 5) to assess the importance of production of somatomedin-C by fetal tissues in vitro.