The transport of L-arginine has been characterized in Chinese hamster ovary cells (CHO). In the absence of Na+ the influx of the amino acid decreased. Both in the presence and in the absence of Na+ L-arginine influx was trans-stimulated and cis-inhibited by cationic amino acids. The amino acid entered CHO cells through an apparently non saturable mechanism and a single saturable agency whose Km increased in the absence of Na+. These results indicate that the agency devoted to transport cationic amino acids in CHO cells resembles system y+, the Na+-independent route that transports cationic amino acids in a number of mammalian models, although its activity is lowered by the replacement of extracellular sodium.