The purpose of this study was to describe the changing histological organization of the rat ovary during postpartum days one through three (p1-p3). A PC-based image-combining microscope system was used to reconstruct the ovary in three dimensions. On p1, cyclindrical pocket-like structures radiated from the core of the ovary that were open toward the surface epithelium. The walls of the pockets contained connective tissue cells and capillaries (stroma). By p2, these pockets had completely closed; each pocket enclosed a small nest of oocytes and a few presumptive granulosa cells. By p3, the pocket-like organization had disappeared. On p1, only one or two primordial follicle-like structures were observed in the core and toward the periphery of the ovary; most oocytes were not enclosed in follicles. By p3, very few naked oocytes remained; primordial follicles predominated in all the regions of the ovary and some of the follicles had multiple layers of granulosa cells. There were changes in location, area, and volume of the rete tubules during these postnatal days. The extraovarian rete was visible on all 3 days but changed its orientation relative to the ovary. The connecting rete was found beneath the epithelial layer of the ovary on all 3 days and showed dramatic increase in area on p2. The wide lumen of the intraovarian rete was in direct contact with some of the oocytes near by on all 3 days, but these "communication points" were most abundant on p2. Based on our observations of different cell-cell associations during this time period, we hypothesize (1) that the mesenchymal-presumptive granulosa cell association is essential for the completion of folliculogenesis, and (2) the rete ovarii may have an inductive role in follicle assembly. These observations suggest that the first 3 days postpartum are critically important for studying the heterogeneous cell interactions that lead to the assembly of primordial follicles. The regional differences in tissue organization during this formative period may have significant implications on later aspects of follicular development.