The histogenesis, the time of origin and the pattern of migration of the cells in the hippocampus and dentate gyrus, have been studied in normal and reeler mice. The earliest indication of a defect in the reeler hippocampus is seen on the fifteenth embryonic day (E15) which is at least 24 hours after the first indication of a defect in the neocortex. It is not until E18, that the dentate gyrus shows signs of its incipient abnormality. It appears then, that in both the hippocampus and the dentate gyrus the gene defect first manifests itself at the stage at which the definitive cellular layers are assembled. Experiments involving the injection of 3H-thymidine (3H-TdR) at different developmental stages have confirmed that the site and rate of cellular proliferation in the reeler hippocampus and dentate gyrus are normal, as is the initial pattern of cell migration. However, in the reeler dentate gyrus, most postnatal cell proliferation occurs ectopically and in the hippocampus the normal "inside-out" sequence of neurogenesis is reversed, the earliest pyramidal cells generated coming to lie superficially within the stratum pyramidale and the later formed cells being added at progressively deeper levels. There is no discernible gradient in the time of origin of the granule cells in the radial dimension of the reeler dentate gyrus, whereas there is an obvious "outside-in" gradient in the normal animal. The characteristic gradients in cell proliferation seen in the transverse and longitudinal dimensions of the normal dentate gyrus are, however, also evident in the reeler mouse. Taken together, these observations suggest that the reeler gene exerts its effect on neuronal position only in the radial dimension, and does so at a stage of development subsequent to the proliferation and initial migration of the relevant neurons. Timm's sulfide silver preparations indicate that the characteristic staining patterns seen in the dentate gyrus and hippocampus appear at the same time, and mature at the same rate in normal and reeler mice.