During histogenesis of the neocortex, Cajal Retzius cells in the marginal zone express the glycoprotein reelin which is developmentally regulated and involved in the formation of the inside out mode of cortical layering. Cajal Retzius cells are also present in the developing hippocampus. There, inhibition of reelin by blocking with CR-50, an antibody which recognizes the N-terminus of this protein, leads to abnormal development of layer-specific connections. Here we report the developmental distribution pattern of reelin expressing neurons in the rat hippocampal formation using CR-50 immunocytochemistry. Labelled Cajal Retzius cells were located near the hippocampal fissure in neonate rats. Many of these cells were still present in the adult. From postnatal day 4 on, neurons in other layers were stained with the CR-50 antibody. In adult rats immunopositive neurons were found in all hippocampal subfields and in the entorhinal cortex. These observations indicate that in the rat hippocampal formation reelin is expressed in different neuronal types during development and in adulthood. Moreover, Cajal Retzius cells in the marginal zone near the hippocampal fissure are still found in adult animals.