In this study the incorporation of [14C]leucine into albumin and transferrin in early rat fetuses, vitelline plus amniotic membranes, chorioallantoic placenta and perinatal rat liver slices, was measured and used to detect and compare the rates of synthesis of the two proteins. Albumin synthesis was detected in the body of fetuses from 13 days gestation onwards. Transferrin synthesis was detected only after day 15. Transferrin synthesis was demonstrable in the membranes but not in the chorioallantoic placenta of all the animals investigated, i.e., from 13 to 19 days gestation. Synthesis of albumin and transferrin by the liver of near term and postnatal animals was shown to correlate with published data on the parenchymal cell number/unit wet wt. of liver. Near term fetuses synthesized relatively more transferrin than albumin when compared with 10 day postnatal animals. The serum concentrations of the two plasma proteins were also determined. These increased before term whereas the rate of synthesis of albumin and transferrin declined. Postnatally, plasma albumin concentration increased but transferrin concentration decreased, yet the rates of synthesis of both proteins by the liver increased with age. This lack of correlation between the rates of synthesis of the two proteins and their respective plasma concentrations could be explained in part by their increased stability after birth. There was also evidence that the liver hemopoietic cells took up transferrin although they do not synthesize the protein. Thus the decrease in this population of cells during development could also contribute to the discrepancy between liver synthesis and serum concentrations of transferrin.