The uptake of 3H-noradrenaline into reserpine-pretreated PC-12 cells (a clonal cell line which possesses "uptake1") was abolished when at high extracellular Cl- all the extracellular Na+ was replaced by Tris+ and when at high extracellular Na+ all the extracellular Cl- was replaced by isethionate. Increases in the external Cl- concentration (at a fixed high Na+ concentration) progressively increased the uptake of 3H-noradrenaline. The same was found with increase in the external Na+ concentration (at a fixed high Cl- concentration). From the anions tested only Br- and SCN- were able to partially mimic the transport-stimulating effect of Cl- (with about 40% and 20% effectiveness, respectively). When chloride was replaced by nitrate or larger anions such as sulphate, methylsulphate or isethionate, virtually no transport of 3H-noradrenaline was observed. The initial rate of uptake of 3H-noradrenaline showed saturation with increasing concentrations of noradrenaline when determined at several fixed concentrations of either Na+ or Cl-. The apparent Km for noradrenaline transport (KmNA) progressively decreased and the VmaxNA increased with increases in the concentration of Na+ (at a high concentration of Cl-) or Cl- (at a high concentration of Na+). The stimulation of the initial rate of uptake of 3H-noradrenaline by increasing concentrations of either Na+ or Cl- obeyed saturation kinetics when determined at several concentrations of noradrenaline. The concentration of Na+ (or Cl-) which caused half-maximal stimulation of uptake (i.e., the apparent KmNa+ and the apparent KmCl-) decreased with increases in the concentration of noradrenaline. These results strongly suggest that Na+ and Cl- are co-transported with noradrenaline.