The present study addressed the hypothesis that there exist multiple sites/states associated with the dopamine (DA) transporter ligands. The authors used [3H](1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine) (GBR12935) and [3H]-N-(1-(2-benzo[b]thiophenyl)cyclohexyl)piperidine ([3H]BTCP) to label binding sites present in striatal membranes and conducted experiments under nearly identical assay conditions, i.e., 18- to 24-hr incubations at 4 degrees C in 55.2 mM sodium phosphate buffer, pH 7.4, with a protease inhibitor and antioxidant cocktail. To obtain data suitable for quantitative curve fitting, it was necessary to repurify the [3H]ligands periodically by high-performance liquid chromatography. Under these conditions, greater than 90% specific binding was observed. The method of binding surface analysis was used to characterize the interaction of GBR12935, BTCP, mazindol and 2 beta-carbomethoxy-3 beta-(4-fluorophenyl) tropane with binding sites labeled by the [3H]ligands. Nonlinear least-squares curve fitting of the data to one- and two-site binding models demonstrated that, for both [3H]ligands, the two-site model fit the data far better than did the one-site model. The results indicated that [3H]GBR12935 labeled two binding sites, with higher (GBR site 1) and lower affinity (GBR site 2) for BTCP (Ki values of 5.84 nM and 1394 nM). [3H]BTCP labeled two sites with high affinity (Kd values of 9.3 nM and 6.3 nM) at which GBR12935 also had high affinity (Ki values of 8.9 nM and 0.98 nM). Intrastriatal 6-hydroxy-DA lesions decreased the density of both [3H]GBR12935 binding sites but affected site 1 much more than site 2, which indicated that a greater portion of GBR site 1 is localized on striatal nerve terminals. By contrast, the 6-hydroxy-DA lesions decreased BTCP site 2 without significantly decreasing BTCP site 1, which indicated that BTCP site 1 is not located on DA nerve terminals. The i.c.v. administration of 5,7,-dihydroxytryptamine did not decrease GBR site 1 or 2, which indicated that neither is located on serotonin striatal nerve terminals. Viewed collectively with other reports, these data support the hypothesis that DA transporter ligands label multiple binding sites in caudate membranes. The identification of selective agents for these sites may be valuable tools for identifying drugs that might modulate the effects of cocaine.