1. Factors governing hepatic utilization of alanine were studied in vivo and in vitro in rats adapted to increasing dietary protein. 2. Hepatic alanine utilization was enhanced 5-fold with a 90%-casein diet, compared with a 13%-casein diet. The increased uptake resulted from enhanced fractional extraction in the presence of high concentrations of alanine in the portal vein. 3. The increase in alanine metabolism on high-protein diets was associated with an increase in alanine aminotransferase and in pyruvate utilization for gluconeogenesis. 4. The emergence of a high-affinity component appeared to be responsible for the enhanced transport of alanine with high-protein diets. 5. High extracellular concentrations after alanine loads resulted in a maximal rate of utilization and of accumulation of alanine by liver cells in vivo and in vitro. Alanine accumulation was particularly active with high-protein diets. 6. In starved rats, alanine transport was also increased, but low concentrations of alanine in afferent blood contributed to make transport limiting for alanine utilization. 7. In fed rats, the rates of transport and catabolism of alanine generally appear to undergo parallel changes; both processes thus play a fundamental role in the control of alanine utilization by the liver.