Foetal rats were exposed to 100 r (acute X-irradiation) at 15$\cdot $5, 16$\cdot $5 and 17$\cdot $5 days of intrauterine life, and killed at daily intervals up to 20$\cdot $5 days p.c. The populations of normal and degenerating germ cells in irradiated ovaries were estimated by the volumetric method of Beaumont & Mandl (1962). The chromosomal configurations of germ cells were examined in both histological and squash preparations. Irradiation appears to disturb the close developmental synchronization of germ cells. Many irradiated cells also show transient changes in chromosomal morphology, and an increase in cell and nuclear volumes. The treatment induces both immediate cell death (i.e. within 24 h of exposure) and delayed cell death. The latter becomes manifest by an increase in the incidence of degenerating cells of the same type as arise spontaneously during normal development ('atretic divisions'; 'Z' cells). The most severe depletion in the total population of germ cells occurs following exposure at 15$\cdot $ 5 days, only some 13000 germ cells being present 24 h later (cf. 37000 in coeval controls). Their number is further reduced to about 5000 at 20$\cdot $5 days (cf. 56000 in controls). In contrast, the mean populations at 20$\cdot $5 days following irradiation at 16$\cdot $5 and 17$\cdot $5 days are ca. 16000 and 42 500 respectively. Most degenerating germ cells are eliminated from the ovary by 20$\cdot $5 days. The subsequent loss of oocytes with age (up to 25 and 100 days p.p.) occurs at a lower rate in irradiated than in normal animals.