Using antibodies to neuropeptide Y (NPY) in combination with immunohistochemical techniques we have studied the distribution of cell bodies and nerve terminals containing NPY immunoreactivity (-i) in the hippocampal region of rats and monkeys (cynomolgus). In colchicine-pretreated rats a large number of NPY-positive cells are present in all areas of the hippocampal region. The NPY-i cells range in size from small (diameter across soma: 10-15 micron) to large (approximately 20 micron). Most of the NPY-i cells are situated in the hilus, in the subgranular zone of the area dentata, and in the stratum oriens of Ammon's horn. A majority of these are polymorphic cells but cells of different morphology are present in these layers as well. These include small spheroid cells and dentate pyramidal basket cells that are distinct from the polymorphic cells in the subgranular zone. The subicular complex (e.g., the subiculum, pre-, and parasubiculum) and the entorhinal area contain fewer NPY-i cells than the rest of the hippocampal region. In the dorsal parts of the pre- and parasubiculum numerous small cells are scattered throughout all layers, while in the entorhinal area the NPY-stained cells are situated primarily in the deep layers (V and VI). In the ventral part of the lateral entorhinal area large multipolar and bitufted cells are found in layers II-VI. In the untreated monkey brain NPY-positive cells are found in the hilus of the area dentata and in the deep (IV through VI) layers of both the medial and lateral entorhinal area. Fewer NPY-stained cells are present in the subicular complex and in the entorhinal area. In the monkey as well as in the rat, NPY-stained cells are present in the angular bundle and in the alveus. A dense network of NPY-i fibers innervates the entire hippocampal region in both the rat and the monkey. The hippocampal NPY-i preterminal processes are present primarily in stratum moleculare of Ammon's horn and in the outer one-third of this layer in the area dentata. The NPY-positive innervation of the dentate molecular layer is far more prominent in the monkey than in the rat brain. Numerous NPY-stained fibers are scattered in other areas as well. In all retrohippocampal structures, and in particular the entorhinal area, the NPY-i fibers form a massive network that innervates all layers to about the same extent, with the exception of the molecular layer, which is more densely innervated than the other layers.