Inflamed cat and dog gingivae were examined electron microscopically to determine the ultrastructural features of chronic marginal gingivitis and to compare inflamed with clinically healthy gingiva. With increasing degrees of inflammation there was an increase in: the dilatation of blood vessels; the number of intravascular, extravascular and intraepithelial neutrophils; the frequency and depth at which ruptured and degranulated neutrophils were observed; in the width of the intercellular spaces of the crevicular epithelium and in the number of plasma cells and macrophages. There was a corresponding decrease in the number of fibroblasts, collagen fibrils, and desmosomes in the crevicular epithelium. Some blood vessels were fenestrated and some showed concentric laminae of material which resembled and was continuous with the perivascular basal lamina. Breaks up to several microns in width were observed in the epithelial basal lamina and these were often associated with, or occupied by, neutrophils or macrophages. Neutrophils were found in the keratinized masticatory epithelium only in severely inflamed specimens. The striking features of chronically inflamed gingivae—wide intercellular spaces, neutrophil infiltration, cellular degeneration—were essentially those features which distinguish the crevicular region from other areas of the oral mucosa, but were quantitatively much increa