The distribution of cholecystokinin (CCK)-like (CCK-L) immunoreactive cells and nerve terminals was studied in the brains from rats and guinea pigs by using antibodies to the octapeptide cholecystokinin (CCK-8). Analysis of serial horizontal and sagittal sections through the retrohippocampal region in colcicine-pretreated rats revealed a relatively large number of CCK-L immunoreactive cells in the pre- and parasubiculum, subiculum, and the medial and lateral entorhinal area (EA) at all dorsal to ventral levels of the region. In the EA, the CCK-positive cells were scattered in all layers without any clear pattern. Analysis of CCK-positive cells in the retrohippocampal region showed that these cells form a morphologically heterogeneous group. The types of CCK-L immunoreactive cells ranged from small (approximately 10 micrometers) round, ovoid, or fusiform to large (approximately 30 micrometers) multipolar and pyramidal. CCK-L immunoreactive nerve fibers and preterminal processes were unevenly distributed in the retrohippocampal region. The densest innervation was found in the parasubiculum, subiculum, and the ventrolateral entorhinal area. Only a few scattered fibers were detected in the molecular layers of these structures and the outer layers of the presubiculum. Within the EA and CCK innervation indicated a heterogeneous laminar distribution that was densest in layers II and IV of the medial and lateral EA and diffuse in layers I and II. In layer II the immunoreactive nerve terminals encircled the pyramidal cell bodies, while in layers IV to VI and the most ventral part of lateral entorhinal area (LEA) and the transitional area between LEA and piriform cortex the CCK processes were distributed in a netlike fashion without clear relation to the cytoarchitectural characteristics of the area.