To study the events that lead to the formation of primordial follicles, pregnant rats were given continuous infusions of [3H]thymidine (3H-TdR) beginning on Days 14-19 of pregnancy (e14-e19) and continuing for 48-120 h. Ovaries from the pups were collected and plastic-embedded histological sections were prepared for autoradiography. The autoradiographs revealed that within the core of the developing ovary were a large number of cells that remained mitotically inactive (failed to incorporate label) from e14 through the day of birth. These unlabeled cells gave rise to the granulosa cells of the first follicles that formed, were located in the medulla of the ovary, and were the first to begin growth. The unlabeled cells did not appear to contribute to the formation of the follicles that formed later in the cortical region of the ovary. When 3H-TdR infusion was begun during late pregnancy, a small subset of the germ cells incorporated label, although the vast majority did not. The labeled germ cells are presumed to represent those that were lagging in their development (had not yet entered meiosis). After ovarian histogenesis was completed during the first week postpartum, the unlabeled ocytes were found concentrated in the core of the ovary, enclosed in the earliest growing follicles; labeled oocytes were found exclusively in the cortex of the ovary, within tiny, quiescent primordial follicles. These observations provide some empirical support for long-held, but heretofore untested, hypotheses concerning early folliculogenesis: that the first follicles that begin to grow are qualitatively different from the remaining follicles in the ovary and that primordial follicles begin to grow in the order in which they were first formed.