Severe irradiation on head may result functional alterations of central nervous system. In this study, the irradiation effect on the cerebellar cortex following heavy X-irradiation on head was studied ultrastructurally. Radiation was produced with the linear accelerator ML-4MV[Mitshubishi Co.], and rats weighing about 200gm each were exposed their heads within the radiation areas of 30cm x 30cm, under the radiation distance of 80cm, and with the radiation depth of 1.2 cm. Radiation doses were 3,000rads or 6,000rads, respectively. Animals were sacrificed on 6 hours, 2 days or 6 days following the radiation. Under anesthesia, animals were perfused with 1% glutaraldehyde-1% paraformaldehyde solution. Two hours after the perfusion, brain were taken out and refixed over night in the perfusion fixative. Small blocks of cerebellar hemispheric cortices were refixed 2 hours in 2% osmium tetroxide solution. Fixed tissues were dehydrated in alcohol, embedded in araldite mixture, and cut with ultratome. Ultrathin sections were contrasted with uranyl acetate and lead citrate solutions, and observed with electron microscope. The results obstained were as follow : 1. On 6th hour following X-irradiations, many cerebellar cortical neurons showed increased electron densities, more complicated nuclear infoldings, depletion of synaptic vesicles, expansion of astroglial territories, etc. 2. On 2nd day following X-irradiations, many organelle-rich cells such as Purkinje cells and Golgi cells were darkly degenerated. Numerous myelin figures formed by the cisternal fusions of Golgi apparatus or granular endoplasmic reticula were observed. Cytoplasmic processes of activated astroglial cells were expanded around capillaries and between granule cells. 3. On 6th day following X-irradiations, morphology of neuropil and neurones in the cerebellar cortex was generally restored, except the expanded territories of astroglial cells. From the above results, it was concluded that the release ofneurotransmitters and transcapillary leakage of blood substance were occurred on 6 hours after heavy X-irradiations. And severe alterations were produced on 2 day after X-irradiation, but the condition was generally restored on 6th day following X-irradiation.