The effect of partial (70%) hepatectomy on amino acid transport was investigated by measuring the uptake of the nonmetabolizable amino acid analogue alpha-amino-[1(-14C]isobutyric acid in hepatocytes freshly isolated from the liver remnant. In hepatocytes from partially hepatectomized rats, both the influx and the efflux of alpha-aminoisobutyric acid were increased. Quantitative analysis of the relationship between the initial rate of uptake and the substrate concentration indicated that a high-affinity component was responsible for the increased rate of amino acid entry in hepatocytes from partially hepatectomized rats. This component could be detected 90 min after operation, but its Vmax increased with time. The high-affinity component was strictly dependent on sodium and had the properties of a pure A system; its mode of energization partly involved cationic transmembrane gradients. The L system of amino acid transport was not affected after partial hepatectomy. The rapid emergence of a high-affinity amino acid transport system in the liver remnant following partial hepatectomy may play an important role in the regulation of liver regeneration.