In mammals, the hallmark of late ovarian histogenesis is follicle formation. However, fundamental issues regarding late histogenesis of the rat ovary have not been addressed. Accordingly, the objective of this research was to study and establish morphology, histology, and changing histo-architecture of the neonatal rat ovary during follicle formation. Light and electron microscopic observations and three dimensional reconstruction showed that: during follicle assembly, the oocytes in contact with each other became separated by intervening presumptive granulosa cells to form a single layer of cells around each oocyte, the first few follicles were formed during the second 24 hours after birth, follicle formation was nearly completed within the third 24 hours, there was a basal lamina wherever the epithelial cell apposed the mesenchyme, the basal lamina was not present wherever the presumptive granulosa cell apposed the oocyte, the shape of the neonatal rat ovary was similar to that of a cashew-nut, the intra ovarian rete tubules were in close proximity to the newly formed follicles, and there were dynamic changes in the histo-architecture of the neonatal rat ovary. Immunolocalization of collagen type IV, a marker for basal lamina, showed that: collagen type IV was present in the basal lamina of sex cords and follicles, the epithelial compartment changed from a cluster-like organization to a follicle-like organization, all the stages in follicle formation suggested by other researchers were found applicable to the neonatal rat ovary, and follicle formation began at the center of the ovary and later spread toward the periphery. In situ hybridization of mRNA coding for NC1 domain of {dollar}\alpha1{dollar}-chain of collagen type IV showed that: mesenchymal cells might be solely responsible for the synthesis of collagen type IV before and during follicle formation, presumptive granulosa cells might start synthesizing this protein only after they are assembled into a single layer of granulosa cells and bound by an enveloping basal lamina, and expression of this mRNA coding by granulosa cell might be a marker for commitment of the undifferentiated presumptive granulosa cell to the granulosa cell lineage.