AbstractMorphogenesis of the entire hippocampal region was examined in normal rats from embryonic (E) day E10 to E22 and on postnatal (P) days P1, P7, and P21, and was correlated with autoradiographic datings (Bayer, '79c). This region is postulated to form from two connected primordia in the telencephalon, easily recognizable on E16. One lies in the dorsomedial wall and generates a portion of the subiculum, Ammon's horn and the dentate gyrus. The other lies in the dorsoposterior wall and generates the entorhinal cortex and part of the parasubiculum and presubiculum. A cortical plate appears in the presumptive entorhinal region on E17; a cell‐sparse fibrous zone appears in the middle of the cortical plate on E19. Lamination in the entorhinal cortex proceeds from lateral to medial and from deep to superficial, with the thickening of layer III being the last to appear on E22. Lamination in the subiculum starts on E18, but the distinction between superficial laminae in the para‐ and presubiculum cannot be made until E22. The stratum pyramidale is small on E20 in presumptive Ammon's horn, then greatly lengthens between E21‐P1. In early dentate gyrus development, cells migrate from the neuroepithelium near the outgrowth of the fimbria and adjacent to the choroid plexus toward a subpial location. The ectal limb of the granular layer appears at the two extremes of the dentate gyrus on E20; the endal limb apears perinatally and develops rapidly to become morphologically similar to the ectal limb by P7.A volumetric analysis of growth in Ammon's horn (including a portion of the subiculum), dentate gyrus, fimbria, and fornix was made from E15 to P21. The neuroepithelium increases to a peak volume on E18 and disappears by P1. The subependymal zone becomes distinct on E18, reaches a peak volume on E20 and disappears by P7. Rapid rates of growth in the stratum oriens and strata radiatum, lacunosum‐moleculare occur between E16‐E17 and between E22‐P1. The pyramidal layer grows rapidly between E18‐E19 (presumptive subicular pyramids) and between E22‐P1 (presumptive Ammon's horn pyramids). Growth rates of the dentate hilus are rapid prenatally and decline postnatally, while dentate granular and molecular layers maintain high postnatal growth rates. The fimbria and fornix have early (E18‐E19) and late (E21‐E22) spurts of growth.To accurately locate regions of primitive migratory and mitotic cells within each lamina of Ammon's horn and the dentate gyrus, the number of cells surviving a single exposure to 200 R X‐rays in embryonic brains (E15‐E22) were compared with controls. The neuroepithelium increases its radioresistance from E15‐E21 and reaches control levels by E22; the subependymal zone is highly radiosensitive throughout development. In contrast, radioresistance in the stratum oriens and dentate hilus declines and reaches a low point on E20. Over 70% of the cells in the strata pyramidale, radiatum, lacunosum‐moleculare, and dentate granular layer are radioresistant throughout development.