Evidence has been presented recently which suggests that the sodium ion is essential for the active transport' of a variety of organic solutes. This requirement was first demonstrated by Riklis and Quastel (1) who showed that Na+ was needed for the active transport of sugars by guinea pig intestine. Subsequently, Csaky and Zollicoffer (2, 3), using rat and toad intestinal preparations, reported that Nat was required for the active transport of the monosaccharides glucose and 3-0-methylglucose, the amino acids L-tyrosine and DL-phenylalanine, and the pyrimidine, uracil. Crane, Miller, and Bihler (4, 5) also observed an obligatory Na+ requirement for active sugar transport by intestinal epithelium, and more recently, Kleinzeller and Kotyk (6) reported a similar requirement for the active transport of galactose by renal tubular epithelium.The purpose of this study was to examine the effect of Na+ on a nonactive mediated transport system as well as on an active transport process. Striated muscle was selected for this purpose, since in this tissue, in contrast to intestine and kidney, glucose is transported by a nonactive mediated system (7). Amino acids, however, are transported by both active and nonactive mediated systems and a process characterized by the kinetics of passive diffusion (8,9). Furthermore, both