In this study the incorporation of [14C]leucine into rat liver albumin, transferrin and total soluble protein was measured in animals in the immediate postnatal period. The rate of labelling of total plasma protein was also determined. Administration of the label to surgically delivered rats by intravenous or by intraperitoneal injection gave very similar results. Incorporation of [14C]leucine into all the proteins studied was approximately ten times greater in rats injected intraperitoneally 5-12 h after birth than in animals injected immediately after delivery. Furthermore, in the 5 and 12 h old animals the levels of [14C]leucine incorporated into the proteins in the liver reached a maximum within 20-30 min of injection, whereas in the immediate postnatal rats these levels continued to increase for at least 60 min. In the latter group of animals free [14C]leucine concentrations were higher in the liver and serum than in those studied 5 h after delivery.