We seek to differentiate and describe the several different membrane transport systems for amino acids of various charge distributions, including alpha, alphazwitterionic, anionic and cationic amino acids, in fresh suspended rat hepatocytes, adult and fetal hepatocytes in primary culture, established hepatoma cell lines, human skin fibroblasts, and other cells, with the purposes of understanding the relative cellular and organismal roles, origins, and regulation of each system, and the chemical basis of the recognition and subsequent steps in transport across the plasma membrane. In particular we want to interpret the roles of transport Systems ASC and N, and other incompletely defined systems, including those for anionic and cationic amino acids, in the maintenance of physiological steady states through kinetic and regulatory effects. From the information gained we hope to contribute to the understanding of how the several transport systems function in normal and pathological metabolism.