Objective:To explore the pathology and pathogenesis of cerebellar injuries induced by methylmercury chloride(MMC) toxication in rats. Methods:Rats were given MMC(4 mg·kg-1·d-1) consecutively and sacrificed on days 11, 15, 18 and 21. Pathological changes of the cerebellum were observed by histo-immunopathology; in situ staining was performed for DNA strand breaks in cerebellar granule cells by TUNEL technique; and the ultrastructures were observed by electron microscope. Results:On day 18, sparse TUNEL positive granular cells were observed mainly in deep lamina adjacent to the white matter. On day 21, apoptotic cells markedly increased and granule cells decreased with well-preserved Purkinje cells. Immunostaining with MRF-1 and GFAP demonstrated severe microgliosis and astrocytosis. On day 18, electron microscopy demonstrated that the nuclei of MMC-treated animals were shrunken and displayed increased electron density, and some homogeneously dense nuclear chromatin with tear-drop features, which were compatible with the apoptotic changes. Conclusion:These results indicate that the pathological changes in the cerebellum in this subacute MMC intoxication model resemble human cases, and the degeneration of granule cells is apoptosis.