The ovaries of foetal and neonatal rats (14$\cdot $5 days post coitum to 2 days post partum) have been studied in order to correlate morphological changes in oogonia and developing oocytes with fluctuations in their numbers. The chromosomal configurations of germ cells at different developmental stages were examined in squash preparations. A volumetric method was devised for the separate estimation of the populations of normal and degenerating germ cells. Counts were made of the numbers of mitoses in germinal and somatic cells up to 19$\cdot $5 days p.c. Oogonia are mitotically active up to 17$\cdot $5 days p.c. The onset of the leptotene phase of meiosis in the majority of oocytes (17$\cdot $5 to 18$\cdot $5 days p.c.) coincides with a sharp decline in the number of normal oogonial divisions. The duration of the zygotene phase appears to be considerably shorter (19$\cdot $5 to 20$\cdot $5 days p.c.) than that of pachytene (range 20$\cdot $5 days p.c. to 2 days p.p.). Oocytes in the diplotene phase are typically seen at 2 days p.p. The number of germ cells attaining the dictyate or resting phase is small on the first day after birth, but increases on the second. Four different 'waves' of degeneration of germ cells were observed. The first, affecting a small number of oogonia, occurs before the onset of meiosis, and is characterized by pyknosis. The second is characterized by degeneration of dividing oogonia which probably sets in before or at mitotic prophase. Such 'atretic divisions' reach their peak at 18$\cdot $5 days p.c., when the frequency of normal mitotic divisions is relatively low. The third affects germ cells after they have entered meiotic prophase (particularly at the pachytene stage); the chromosomes condense and the cytoplasm becomes eosinophil (referred to as 'Z' cells). The fourth affects germ cells at the diplotene phase. Oogonial mitoses are associated with an increase in the population of germ cells from 12 000 on day 14$\cdot $5 to 71 000 on day 17$\cdot $5 p.c. The appearance of 'atretic divisions' is largely responsible for the reduction of about 3000 in the number of normal germ cells between days 17$\cdot $5 and 18$\cdot $5 Successful mitotic divisions, on the other hand, account for the increase of about 4000 in the total number of germ cells during the same interval. From day 18$\cdot $5 onwards, both the total number and the number of normal germ cells decreases steadily, first due to the occurrence of 'atretic divisions' and later to degeneration of 'Z' cells and atretic cells at diplotene. Thus by 2 days after birth, the peak number of normal germ cells (64 000 at 17$\cdot $5 days p.c.) has fallen to 19 000, the total population (peak 75 000 at 18$\cdot $5 days p.c.) being reduced to 27 000. The majority of germ cells appear to be eliminated from the ovary within 24 h of the onset of degeneration.