Membrane vesicles isolated from untransformed Balb/c and Swiss mouse fibroblasts and from those transformed by simian virus 40 catalyzed carrier-mediated uptake of L-alpha-aminoisobutyric acid. Concentrative uptake required the presence of a Na+ gradient (external Na+ greater than internal Na+) and occurred independently of endogenous (Na+ + K+) ATPase activity. This process is electrogenic, since uptake was stimulated by a K+ diffusion gradient (internal greater external) in the presence of valinomycin or by the addition of the Na+ salt of a permeant ion, conditions expected to create an interior-negative membrane potential. Both the initial rate of concentrative uptake of L-alpha-aminoisobutyric acid and its maximal accumulation, driven by a standard Na+ gradient, were decreased in vesicles from density-inhibited, untransformed cells and increased in those from cells transformed by simian virus 40 compared with vesicles from proliferating untransformed cells. An increased maximal velocity (Vmax) of uptake stimulated by Na+ gradient was observed in vesicles from transformed cells compared with those from untransformed cells, suggesting an increase in the number of carriers or in their mobility. Since the relative extent of accumulation of this model amino acid driven by a standard Na+ gradient also differed with growth or transformed status, an additional possibility for cellular regulation of this process could be alteration of membrane Na+ permeability or carrier response to Na+.