In this paper qualitative investigations by means of light microscopy (Golgi impregnation) considering the ontogenesis of efferent neurons of the CA3, CA4a, CA4b regions in the rat hippocampus are presented following the already described development of CA1 pyramids (MINKWITZ and HOLZ 1975). After the migration of neuroblasts the types of projective neurons can be distinguished in the mentioned regions at the end of the prenatal period. During the first postnatal time particularly the basal dendritic tree develops expressing more distinctly the typical forms of pyramids (CA3, CA4a) and the of the multipolare neurons of CA4b. No distinct qualitative jumps at the transition of the regions in contrast to the CA3--CA1 boundary can be observed. Temporal retardations in the appearance of postsynaptic structures (microdendrites, excrescenses, spines) at pyramidal neurons from CA4 to CA3 and CA1 are regarded as signs of earlier development of the phylogenetic older pool of cells. During the early postnatal developmental period (until stage P10) proceedings of the growth are predominant. Later on, during the late postnatal period, the maturation of the neurons takes place with a considerable multiplication of spines distributed in similar ways as known from other pyramids. At stage P20 neurons of all hippocampal regions cannot be distinguished from adult ones without using morphometric procedures. The development of parts of the intrahippocampal network including mossy fibre system and SCHAFFER collaterals is discussed with regard to in this way appearing boundaries between fascia dentata and regio inferior CA4, CA3) and regio superior (CA1). These fibre systems establish by their stepwise construction one of the basic elements to hippocampal functioning.