The specific uptake of [3H] dopamine (DA) was studied using a crude synaptosomal fraction obtained from rat striatum. In a medium containing a 10 mM NaHCO3/NaH2PO4 buffer and no added K+ ions, addition of NaCl elicited an increase in DA uptake for Na+ concentrations from 10 to 60 mM, and then a decrease of uptake for Na+ concentrations up to 130 mM. These data confirm that rather low NaCl concentrations produce a maximal DA uptake. This biphasic curve of uptake resulted from significant changes in the Vmax of the DA uptake. Except for 10 mM Na+, this curve was not significantly modified when 9 mM NaHCO3/NaH2PO4 were replaced by 9 mM NaCl. This result indicates that the Cl- dependence of the DA uptake is mainly secondary to the Na+ dependence. Addition of KCl up to 3 mM did not modify the ascending part of the NaCl-dependent uptake curve. In contrast, the reduction in uptake produced by high Na+ concentrations was prevented in a concentration-dependent manner by KCl; this effect resulted from a decrease in the Km and an increase in the Vmax for the uptake. Measurements of membrane potential, with the help of the fluorescent probe 3, 3'-diethylthiadicarbocyanine iodide [DiSC2(5)] and purified synaptosomes prepared from rat striatum and cerebral cortex, revealed that addition of 3 mM KCl to a medium containing a high Na+ concentration and no K+ ions produced a marked and stable decrease in the fluorescence level. This decrease which corresponds to an increase in membrane polarization was blocked by 0.1 mM ouabain. These data suggest that low K+ concentrations are likely to prevent the decrease in uptake elicited by high Na+ concentrations by restoration, via a Na+/K+ ATPase-mediated mechanism, of the membrane potential and/or a transmembrane electrochemical Na+ gradient more favourable to DA uptake.