We have confirmed previous demonstrations of sodium gradient-stimulated transport of L-alanine, phenylalanine, proline, and beta-alanine, and in addition demonstrated transport of N-methylamino-isobutyric acid (MeAIB) and lysine in isolated rabbit kidney brush border vesicles. In order to probe the multiplicity of transport pathways available to each of these 14C-amino acids, we measured the ability of test amino acids to inhibit tracer uptake. To obtain a rough estimate of nonspecific effects, e.g., dissipation of the transmembrane sodium electrochemical potential gradient, we measured the ability of D-glucose to inhibit tracer uptake. L-alanine and phenylalanine were completely mutually inhibitory. Roughly 75% of the 14C-L-alanine uptake could be inhibited by proline and beta-alanine, while lysine and MeAIB were no more effective than D-glucose. Roughly 50% of the 14C-phenylalanine uptake could be inhibited by proline and beta-alanine; lysine was as effective as proline and beta-alanine, and the effects of pairs of these amino acids at 50 mM each were not cumulative. MeAIB was no more effective than D-glucose. We conclude that three pathways mediate the uptake of neutral L, alpha-amino acids. One system is inaccessible to lysine, proline, and beta-alanine. The second system carries a major fraction of the L-alanine flux; it is sensitive to proline and beta-alanine, but not to lysine. The third system carries half the 14C-phenylalanine flux, and it is sensitive to proline, lysine, and beta-alanine. Since the neutral, L, alpha-amino acid fluxes are insensitive to MeAIB, we conclude that they are not mediated by the classical A system, and since all of the L-alanine flux is inhibited by phenylalanine, we conclude that it is not mediated by the classical ASC system. L-alanine and phenylalanine completely inhibit uptake of lysine. MeAIB is no more effective than D-glucose in inhibiting lysine uptake, while proline and beta-alanine appear to inhibit a component of the lysine flux. We conclude that the 14C-lysine fluxes are mediated by two systems, one, shared with phenylalanine, which is inhibited by proline, beta-alanine, and L-alanine, and one which is inhibited by L-alanine and phenylalanine but inaccessible to proline, beta-alanine, and MeAIB. Fluxes of 14C-proline and 14C-MeAIB are completely inhibited by L-alanine, phenylalanine, proline, and MeAIB, but they are insensitive to lysine. Proline and MeAIB, as well as alanine and phenylalanine, but not lysine, inhibit 14C-beta-alanine uptake. However, beta-alanine inhibits only 38% of the 14C-proline uptake and 57% of the MeAIB uptake. We conclude that two systems mediate uptake of proline and MeAIB, and that one of these systems also transports beta-alanine.