The properties of branched chain amino acid transport systems in Escherichia coli strains W3092 and WV-5 have been described. W3092 is valine-sensitive and WV-5 is valine-resistant. Various compounds which are structurally related to branched chain amino acids were studied for their abilities to inhibit the growth of cells and to inhibit the transport activity. Among them, DL-α-hydrazinoisovaleric acid was found to strongly inhibit the growth of both strains. The compound, however, did not affect branched chain amino acid transport activities. Acetohydroxy acid synthetase activity was measured under various conditions. In both strains, the enzyme was repressed by valine and leucine and showed similar sensitivity to inhibition by valine. The valine, isoleucine, and leucine transport systems of these strains were characterized as having two different Km values characteristic of the entry of the respective amino acids. All the transport systems showed negative cooperativity. The change induced by valine in the transport systems were analyzed circumstantially. Valine and leucine greatly reduced the level of binding protein specific for branched chain amino acids when the amino acids were added to cultures of both strains. This suggests that valine acted as an effector of the transport systems, altering their Km and Vmax values, and their Hill coefficients. Pollock's view of physiological efficiency was introduced in considering the changes effected by valine. Based on this evaluation, the physiological role and regulation of the transport systems are discussed. The mechanism and regulation of active transport systems for amino acids seem to be directed to retaining amino acids in the cell and maintaining their concentrations against a concentration gradient.