Pregnant WKA/Hok rats were exposed to 100 R or 200 R X-irradiation on one of gestation days 16 through 21. Offspring were killed at 60 days of age and the cerebellum was examined. The cerebellum of animals exposed to 200 R was slightly reduced in weight but not in width. The observed reduction in the dorsoventral length of the cerebellum was more evident when the X-irradiation was early in gestation. The anteroposterior length of the hemispheres increased following exposure to X-ray on days 16 through 19, and that of the vermis and paravermis decreased following treatment on days 17 through 21. Therefore, the anterior portions of hemispheres were situated anterior to the culmen in every 200 R group. Somewhat anteroposteriorly and horizontally directed lobules, as opposed to the normal transverse arrangement, were seen in the cerebellum of rats treated on day 16 or 17. Lobule contortion and fragmentation and an increased number of sublobules were striking in cases treated later. Histologically, ectopic Purkinje cells in the granule cell layer and white matter appeared following X-irradiation on day 20 or 21, but they were not found following earlier treatment. In the cerebellum of animals exposed to 100 R the reduction in size was mild and the folial abnormalities were rare, but the number of sublobules decreased.