The distribution of cholecystokininlike (CCK-L) immunoreactive cells and fibers in the rat hippocampal formation and its afferent and efferent connections was studied using the immunoperoxidase technique. In the hippocampal formation CCK-L immunoreactive perikarya were located in the polymorphic zone of the dentate hilus, all layers of Ammon's horn, the subiculum, the presubiculum, and the entorhinal cortex. Cholecystokininlike immunoreactive fibers extended from cell bodies or were located around the cell bodies in the entorhinal cortex, subiculum and stratum pyramidale of Ammon's horn, and among the granule cells and inner molecular layer of the dentate gyrus. The immunoreactive cells in the stratum oriens may be a type of basket cell, since processes from these cells extend into stratum pyramidale and collections of CCK-L immunoreactive fibers are seen around cell bodies in stratum pyramidale. Cholecystokininlike immunoreactive fibers were also observed in the alveus, ventral and lateral fimbria, and ventrolateral lateral septal nucleus. Some of these immunoreactive fibers, therefore, being to either an efferent or afferent hippocampal pathway(s) originating from CCK-L immunoreactive pyramidal cells in the hippocampal formation and/or from the hippocampal subcortical nuclei, the supramammillary nucleus, and the dorsomedial hypothalamic nucleus which contain CCK-L immunoreactive perikarya. The distribution of these immunoreactive fibers in the fimbria and lateral septal nucleus is most consistent with an anteriorly directed efferent hippocampal pathway.