The purpose of the present study was to elucidate regional differences in the vulnerability of cerebellar foliations of rats exposed to X-irradiation. Effects of X-irradiation on foliations were examined with respect to histological changes in Purkinje cells and Bergmann glial fibers by calbindin-D28k (CB) and glial fibrillary acidic protein (GFAP) immunohistochemistry, respectively. Wistar rats were exposed to X-irradiation (1.5 Gy) on postnatal day (PND) 1. At 3 weeks of age, the cerebellum was examined. The cerebella of rats exposed to X-irradiation showed smaller and abnormal foliations compared with controls. Fewer cerebellar foliations due to fusion with neighboring folia were observed in folia I-III and VIa-VII. Moreover, the extent of such abnormalities was more severe in the latter folia. CB-immunoreactive (IR) Purkinje cells exhibited thin, short, disoriented dendrites that had invaded the granular layer or white matter. On the other hand, GFAP-IR Bergmann glial fibers had not extended their processes into the molecular layer perpendicular to the pial surface, and they appeared thin and disoriented. Accordingly, the above cerebellar abnormalities were more severe in folia I-III, VIa-VII and X than in other regions. In contrast to the histological alterations in these folia, there were no apparent qualitative differences in folia IV-V between X-irradiated and controls. These findings indicate regional difference in the vulnerability of cerebellar folia to X-irradiation. Such differences might be attributed to the cerebellar neurogenetic gradient.