The cellular localization and neurochemical phenotype of cells expressing the GAT-1 GABA transporter was investigated in the adult rat dorsal striatum using single and dual in situ hybridization and immunocytochemical techniques. Cellular sites of GAT-1, GAD67, and parvalbumin mRNAs were visualized using a combination of radioactive and alkaline phosphatase-labeled oligonucleotides and emulsion autoradiography; GAD67 immunoreactivity was detected using a polyclonal antibody (K2) and 3'3"-diaminobenzidine. Two types of GAT-1-positive striatal cells were detected: (1) those expressing an abundance of GAT-1 mRNA, and (2) those expressing low/undetectable amounts of message. This study focused on the striatal cells expressing an abundance of GAT-1 mRNA; these cells accounted for approximately 3-5% of all striatal neurons and were detected scattered sparsely throughout the striatal complex. Dual in situ hybridization and immunocytochemical studies established that all cells enriched in GAT-1 mRNA also expressed high levels of GAD67 mRNA and were strongly GAD67 immunopositive; the converse was also found to be the case, the two hybridization signals having identical distribution patterns. Further dual in situ hybridization studies established that approximately 60% of these high GAD67/GAT-1 cells expressed parvalbumin mRNA, a marker of one population of striatal interneurons, and had an average cross-sectional area of 152.40 microns 2. The chemical phenotype of the remaining 40% of high GAD67/GAT-1 cells was not determined, although the average cross-sectional area of these cells (102.48 microns 2) was significantly smaller than GAT-1/GAD67/parvalbumin cells; these cells were detected in all striatal regions and are likely to correspond to another population of striatal GABAergic interneuron.