Mouse erythroid band-3 protein was incorporated into the plasma membrane of Xenopus oocytes by microinjection of poly(A)+-mRNA from spleens of anemic mice. Subsequently, the efflux of microinjected 36Cl was continuously followed in single oocytes in a perfusion chamber the bottom of which was formed by the window of a Geiger-Müller tube. During the flux measurements, the membrane potential was clamped to different holding potentials. The efflux increased over the voltage range of -10 to -100 mV by a factor of about 1.5. Since the membrane potential cannot act as a driving force of anion exchange, it is suggested that the observed slight potential dependence is related to a recruitment of the anion-loaded transport protein by the electrical field, thereby changing the steady-state distribution between inwardly and outwardly facing anion binding sites of the transport molecules. The experimental data are discussed in terms of ping-pong kinetics, assuming that the potential dependence is primarily due to an effect of the electrical field in the membrane on the rate-limiting interconversion of inwardly and outwardly oriented anion binding sites. The results are compatible with the assumption that in the oocyte membrane the substrate-loaded band-3 molecules are preferentially inwardly oriented, and that the transition from the inwardly to the outwardly oriented conformation is associated with a reorientation of an effective charge of 0.1 elementary charge. During progesterone-induced maturation of the oocytes, several endogenous transport systems change their activity drastically. The mouse band-3 protein in the oocyte membrane also undergoes activity changes; however, these changes do not seem to involve direct regulation by specific metabolic processes. They can be explained as a consequence of the depolarization of the membrane potential associated with the maturation process.