Many reports have been made on the dehydrogense acti vity of human gingivae, and some on this activity on gingiva of guinea pigs infected with oral bacte ria. But very few investigations have ever been made into the dehydrogenase activity on the inflammatory cells of human gingivae and on the gingivae of guinea pigs which had been injected several times with oral bacteria. The materials used for the experiment were a piece of gingiva exci sed from patients at our hospital at the first examination and after antiphlogistic treatment, who was suffering from marginal periodontitis, and a piece of gingiva of guinea pigs inoculated with oral bacteria. These gingivae were used as a cryostat serial section, and each dehydrogenases of LDH, NADMDH, NAD-ICDH, GDH, ADH, β-HBDH, α-GDH, NADP-MDH, NADP-ICDH, G6PDH, 6PGDH, NADHDH, NADPHDH, and SDH was detected by the method of Barka and Anderso n and observed by means of an optical microscope, while a part of the inflamed human gingivae was made into a section by means of vibratome and SDH according to Ogawa's method wa s observed by an electron microscope. The following results were obtained:1) In the case of the inflamed human gingivae, the activity of the LDH, G6PDH, NADHDH, and NADPHDH on the inflammatory cells among others was observed a little stronger in the gi ngivae excised at the first examination than the one excised after antiphlogistic treatment. But little difference was found with each dehydrogenases, that is, NAD-MDH, NAD-ICDH, GDH, ADH, β-HBDH, α-GDH, NADP-MDH, NADP-ICDH, 6PGDG, and SDH:2) The electron microscopic examination of the inflamed human gingivae revealed a reaction of SDH with high electron density on the outer compartment of the mitochondria of the lower cell layer of the epithelium.3) In both caces of the gingivae of the guinea pigs, a considerable degree of decrease or disappearance of the almost enzyme activity was observed in the suppurative focus inflicted by an acute inflammation and there was found a little increase of the activity about the respiratory enzy-me system around the suppurative focus. Since the fourteenth day of the experiment, a gradual increase of the enzyme activity on the cells being responsible for the repair of the tissue began to be observed.4) From these findings, the enzyme activity changes in accordance with the degree and process of the inflammation, and the enzyme activity of macrophage and fibroblast being responsible for phagocytosis and proliferation was observed to increase. While the pattern of enzyme activity on cells appearing at the stage of the acute and chronic inflammation has also been established.