Several studies have dealt with the morphogenesis of the rat hippocampal pyramidal cell, but little is known about how the different pyramidal cell shapes of CA fields differentiate from neuroepithelial cells, or about how the field morphological identity emerges. From our studies of pyramidal cell shapes in the CA1, CA3, and CA4 fields of hippocampi at postnatal developmental stages between P0 and P12, using fresh semidissociated slices and acutely dissociated cells, we identified the sequence of cell shape transformation by which they differentiate from simple bipolar to complex shapes characteristic of adult pyramidal cells of CA1 and CA3. Pyramidal cell morphogenesis does not occur synchronously throughout the CA hippocampus fields, but cells in the CA4 field undergo morphological differentiation at earlier stages, prenatally, than CA3 cells, and these in turn earlier than CA1 cells. Thus, during the P1-P6 stages, a gradient of shapes from less to greater differentiation was clearly observed from CA1 to CA4 in a single slice. Furthermore, a mixture of cells at different degrees of differentiation is observed in CA1 from P1 to P10, and in CA3 from P1 to P5. A gradient of shapes from more to less differentiation was observed at stages P5-P6 from septal to temporal. We describe two processes in the pyramidal cell morphogenesis in the CA1 and CA3 fields, the approximation to the soma of the point of bifurcation of the main apical process, and the acquisition of triangular shape of the soma, showing by a quantitative study of both processes that they occur earlier in CA3 than in CA1. Our study, therefore, provides new insight into rat pyramidal cell morphogenesis, and indicates that this process might be differently regulated in the various CA fields. Hippocampus