Two-microelectrode voltage clamp was used to measure membrane currents resulting from flux of cationic amino acids in Xenopus oocytes expressing the cloned dual-function ecotropic murine leukemia virus receptor/system y+ transporter. At membrane potentials ranging from +20 mV to−120 mV, arginine influx obeyed Michaelis-Menten kinetics. At concentrations from 0.01 to 1 mM, influx increased exponentially with membrane hyperpolarization (e-fold increase/−59 mV). Efflux from oocytes preloaded with arginine increased exponentially with depolarization (e-fold increase/+52 mV). Kinetic analysis based on an iso uni uni facilitated transport model suggests that the effect of voltage on steady-state flux arises largely from charge movement across the membrane field during the conformational transition of the unliganded transporter which switches the substrate binding site from one membrane face to the other. This charge movement would facilitate rapid increases in intracellular arginine concentrations in response to hyperpolarization, a property which could play a role in modulating nitric oxide synthesis in some types of cells.