1. The main objective of this work was to investigate the extent of cellular colocalization of dopamine D1 and D2 receptors in the rat brain. A double labeling technique, that combined immunocytochemical labeling of the D2 receptor using polyclonal antibodies raised against the third intracellular loop of the short isoform of the human D2 receptor in combination with in situ hybridization detecting D1 mRNA expression, was designed to accomplish this goal. 2. The specificity of the antisera obtained was confirmed by immunoprecipitation assay, Western blot analysis, and immunocytochemistry on D2R transfected cells and murine brain tissue. Western blot using the D2 receptor antibody revealed a specific broad band centered at 67 kDa in transfected cells and a major protein of 88 kDa corresponding to D2R expressed in the caudate-putamen, to a lesser extent in the cortex, and not at all detected in the hypothalamic region. 3. The content of neurons double-labeled for D1/D2 receptors was observed at in differing intensities in the dorsal endopiroform nucleus, the intercalated nucleus of amygdala, the anterior part of the cortical nucleus amygdala, the nucleus of the lateral olfactory tract, the piriform cortex, the parabrachial nucleus, the supraoptic nucleus and the parabigeminal nucleus. All other regions of the brain revealed neurons expressing either D1 or D2 dopamine receptors but not both at that same time. 4. These results clearly demonstrated that specific neurons expressed both receptors D1 and D2, and that this colocalization was restricted to particular regions of the rat brain.