Damage to periodontal tissues caused by dental materials (silver alloy powder) was examined by histopathological observation. Three kinds of silver alloy powder containing copper of various contents, such as that used in the first group: Ag-Cu (25% Cu), second group: Ag-In (5% Cu), and third group: Ag-Sn-Zn (0% Cu), were used to analyze damage to periodontal tissues caused by silver alloy powder. With No. 1/2 round bar, standardized small cavities were prepared in the mesiolingual cervical portion of the upper first molars of rats. Three kinds of silver alloy powder were then inserted into the small cavities. Postoperative changes in the gingival tissues were studied histopathologically. The results were as follows: 1. Under the light microscope, all three groups showed similar tissue reaction. Many dark deposit (metal) were observed in the lamina propria of the gingiva. These dark deposits had been taken into the histiocyte-like cells and multinucleate giant cells. Infiltration of cells was not observed around these dark deposits. Findings among the metals in three groups were of specific structures, which displayed black and dark-brown stained deposits in surrounding silver alloy powder. 2. Under the electron microscope, the first, second and third groups showed similar tissue reactions. Dark deposits (metal) had been taken into the multinucleate giant cells. These cells contained many lysosomes, vacuoles, and endocytosed materials. The deposits consisted of a dence central core of silver alloy powder surrounded by fine needle-like components and filaments of lower electron density. Extremely fine particles were also observed around the basal lamina of capillary vessels and along collagen fibers. 3. By AEM, components of the lower electron density material were identified as silver, copper and sulfur in the first and second group, and silver and sulfur in the third group. The elements in the fine particles were silver and sulfur. 4. Using scanning electron microscope, the micrograph indicated that a barnacle-like structure was formed on the surface of the silver alloy in the gingival tissue. A phenomenon of solubility appeared as stacks of granules firmly attached to the silver alloy surface. 5. The silver element of the silver alloy powder damaged the gingival tissues slightly, resulting in minor tissue reactions. When the range of copper content was from 0% to 25%, the alloys caused no damage to gingival tissue.