Gingivitis was induced by the accumulation of microbial plaques on the tooth surfaces in beagle dogs. The state of the gingiva was assessed by gingival exudate measurements and biopsies submitted to histological and histochemical analysis. The activity and the distribution of lactate and malate dehydrogenases as well as adenosinetriphosphatase (ATPase) in the gingival epithelium was related to the extent of inflammation as indicated by accumulation of inflammatory cells. The distribution of lactate and malate dehydrogenase activity was identical in healthy gingival specimens and in non-infiltrated areas of subclinically inflamed specimens. The epithelium close to connective tissue areas exhibiting inflammatory cell infiltration possessed increased oxidative enzyme activity. The endothelial cells of the vessels within the inflamed areas also showed markedly increased dehydrogenase activity. In normal gingiva, no ATPase activity was observed in the crevicular epithelium, whereas in epithelial cells close to infiltrated connective tissue compartments ATPase activity was regularly observed. In the oral epithelium, ATPase activity was found within the prickle cell layer in infiltrated as well as non-infiltrated specimens. It is suggested that the enhanced activity of the enzymes is related to increased metabolic activity of the epithelial and endothelial cells, probably as a result of the inflammatory process in the underlying connective tissue.