The studies reported here were carried out to characterize further previously described changes in membrane localized amino acid transport associated with simian virus 40 transformation of the mammalian cell line, Balb/c3T3. Membrane vesicles were prepared from confluent cultures of both simian virus 40 transformed Balb/c3T3 (SV3T3) and the untransformed parent line, Balb/c3T3 (3T3). An initial, externally imposed out greater than in, 100 mM a+ gradient produces acceleration of early ingress of alpha-aminoisobutyric acid (AlB) in vesicles from both cell lines, but transient, concentrative uptake (overshooting) only in SV3T3 vesicles. Early ingress of L-leucine is also accelerated in SV3T3 vesicles by a Na+ gradient, and overshooting is also demonstrable. Na+-gradient independent AIB permeability of SV3T3 and 3T3 membranes was estimated using uptake data, a first order rate equation and measurements of vesicle size derived from quasi-elastic light-scattering studies. AIB permeability of SV3T3 membranes is greater than that of 3T3 membranes (113 A/min and 43 A/min, respectively), suggesting that overshooting in 3T3 vesicles is not attenuated by a Na+-independent AIB "leak". Na+ permeability of the two membranes is similar, ruling out the possibility that a slower rate of Na+ equilibration across the SV3T3 membrane allows development of the overshoot. In SV3T3 vesicles the height of a Na+-gradient dependent overshoot varies with the initial [Na+]o/[Na+]i ratio, and ln[Na+]o/[Na+]i is linearly related to ln AIB uptake at overshoot peak/AIB uptake at equilibrium, consistent with the possibility that for [Na+]o/[Na+]i ratios in the range studied, AIB overshoot is energized by a constant proportion of the energy available from the initial electrochemical gradient for Na+. These results are consistent with the possibility that Na+-gradient dependent overshooting in SV3T3 vesicles is produced by Na+-amino acid carrier interactions resulting in either an increase in maximum transport velocity or an increase in carrier affinity for AIB.