The distribution of dopamine (DA) D1 and D2 receptors in the hippocampal region was studied using in vitro receptor autoradiography with 125I-SCH 23982 and 125I-NCQ 298, respectively. The specific binding of both ligands indicated the existence of D1 as well as D2 receptors in the rat hippocampal region. A closer analysis revealed, however, a different laminar distribution of the two receptor subtypes: layers with a high density of one DA receptor subtype had low density of the other. Thus, in the entorhinal cortex, which contained the highest densities of both subtypes, the following pattern was seen. Layers 2, 4, 5 and 6 had high densities of D1 receptors and low densities of D2 receptors while layers 1 and 3 had high densities of D2 receptors and low densities of D1 receptors. The parasubiculum contained D1 receptors but not D2 receptors and the presubiculum had D2 receptors in layer 2 but few D1 receptors. Similar patterns were recorded in Ammon's horn: the part of stratum lacunosum-moleculare which was rich in D1 was poor in D2 receptors. The interdigitating laminar distribution of D1 and D2 receptors suggests that the effects of DA are mediated via D1 and D2 receptors located at different levels along the intrinsic hippocampal circuit.