The mechanism of gamma-aminobutyric acid translocation in synaptic plasma membrane vesicles from rat brain has been probed by comparing the ion dependency of net efflux with that of exchange. Furthermore the question has been asked if the same mechanism operates for other solutes translocated by this transporter. Dilution-induced efflux of gamma-aminobutyrate from the membrane vesicles is about 3-fold stimulated by externally added gamma-aminobutyrate. Half maximal stimulation is obtained at a gamma-aminobutyrate concentration similar to the Km for gamma-aminobutyrate influx. This stimulation (exchange) is dependent on external sodium but not on external chloride. In contrast to this gamma-aminobutyrate influx is absolutely dependent on the simultaneous presence of sodium and chloride ions (Kanner, B.I. (1978) Biochemistry 17, 1207-1211), while efflux is dependent on the presence of these two ions on the inside (Kanner, B.I. and Kifer, L. (1981) Biochemistry 20, 3354-3358). Nigericin stimulates dilution-induced efflux of gamma-aminobutyrate from potassium loaded vesicles to a larger extent than external gamma-aminobutyrate. gamma-Aminobutyrate further enhances the nigericin-induced stimulation, provided that the vesicles are not preloaded with chloride. Nipecotic acid is transported with the same features as gamma-aminobutyrate and the two solutes behave similar with respect to the ion dependence of net flux and exchange. A model for the translocation cycle is proposed in which at least one of the translocated sodium ions binds to the transporter in its 'outside' conformation after chloride and the solute have bound previously. Conversely, the solute is released from its 'inside' conformation prior to chloride and at least one of the sodium ions.