The mechanisms responsible for the increase in incorporation of radio-active amino acid into albumin, transferrin and total soluble liver protein which occurs in the immediate postnatal period in the rat was investigated in rats delivered surgically in the last 2 days of gestation. The in vivo incorporation of [14C]leucine into the proteins in the liver was low at birth, but increased rapidly during the first half hour after delivery and then more gradually during the subsequent 4.5 h. Neonatal adrenalectomy had no effect on this pattern of results. Intraperitoneal administration of an amino acid supplement had little effect on [14C]leucine incorporation immediately after birth but increased incorporation at 0.5 h and eliminated the second phase of rising incorporation values between 0.5 and 5 h. The in vitro incorporation of 14 C into albumin, transferrin and total protein by slices of the liver from animals immediately after delivery was as great as with slices from animals 5 h after delivery. It is concluded that the initial increase in synthesis of proteins in the liver in the first 0.5 h after delivery is probably due to an increase in the supply of metabolic energy due to improved oxygenation of the rats and that the slower increase in protein synthesis between 0.5-5.0 h results from an improved supply of amino acids to liver cells. It is unlikely that changes in the secretion of adrenal hormones is involved.