Uptake of L-arginine and L-lysine into brush border membrane (BBM) vesicles from rats fed either a high protein (HP) or a high carbohydrate (HC) diet was studied under conditions of a transmembrane Na+-gradient (Na+out greater than Na+in) or a Na+-equilibrium (Na+out = Na+in). The Na+-gradient caused a stronger stimulation of basic amino acid transport across the BBM in group HP than in group HC. Replacing Cl- as counter ion of Na+ by SCN- in the Na+-gradient experiments did not affect L-lysine uptake. In one series of experiments inhibitory effects of several amino acids and D-glucose on Na+-gradient-dependent L-arginine transport into BBM vesicles were tested. L-Lysine showed the strongest inhibition of L-arginine uptake, but also L-leucine, L-alanine and D-glucose caused a significant decrease of L-arginine uptake into BBM vesicles. The main conclusions are that a transmembrane Na+-gradient is able to energize the transport of basic amino acids across the intestinal BBM and that the transport mechanism of the intestinal BBM for basic amino acids adapts to the protein content of the diet.