Cationic amino acid transport (arginine and lysine) into bovine mammary tissue occurs by a sodium independent and saturable mediated system. Concentrative uptake ratios (cell concentration/media concentration) for both arginine and lysine varied between 6 and 22. High concentrations of specific inhibitors of the neutral amino acid transport systems had no effect upon arginine or lysine uptake. Both arginine and lysine were strong inhibitors of each others uptake, whereas ornithine showed less specificity for inhibition of arginine and lysine uptake. In the presence of all amino acids, cationic amino acid uptake occurred at a rate equivalent to that with cationic substrates alone. The presence of the independent cationic amino acid transport system in bovine mammary tissue accounts for the high cationic amino acid uptake by measuring arteriovenous differences of plasma across the bovine mammary gland. This transport system is responsible for excess uptake (beyond milk protein synthesis requirements) of cationic amino acids for catabolism to other amino acids or potential oxidation.