Peculiarities of neutral amino acid (L-leucine and L-phenylalanine) transport in brush-border membranes of rat small intestine enterocytes under normal conditions and following a 1.0-Gy X-ray irradiation have been studied. The increase of the brush-border membrane permeability for ions and amino acids is considered to be the main reason of the post-irradiation disorders in the transmembrane transport of amino acids. The radiobiological approach made it possible to corroborate the existence of a Na+-dependent L-phenylalanine transport system different from the common system for neutral amino acids.