The changes in the ultrastructure of fibrillar centres, morphological type of nucleolus, and the content of Ag-nucleolus protein of Sodium Butyrate treated MGC-803 cells have been studied. It was found that the number of fibrillar centres and Ag-staining granules, content of Ag-staining protein of nucleolus were decreased, and nucleolar morphological type was changed from reticulated to ring-shaped in NaBT treated MGC-803 cells. These results indicate that NaBT regulate the proliferation and differentiation of cells by inhibiting rRNA synthesis and rDNA transcriptional activity.