Abstract A group of seventy-five rats were fed synthetic diets in order to test the effect of zinc poisoning on suboptimal pantothenic acid diets. Weanling rats, weighing 40 ± 3 Gm. were chosen. Forty-seven of these were fed a synthetic diet, complete in every respect but suboptimal in pantothenic acid (about one-fourth of minimum requirements) and containing 0.8 per cent zinc as zinc carbonate; eighteen served as littermate controls; an additional ten rats were fed a diet totally deficient in pantothenic acid. The oral cavities of most were examined at frequent intervals and forty-three of the entire group were studied histologically. The histologic examination included the tongue, cheeks, palate, teeth, and jaws. Other tissues studied were the adrenal glands, kidneys, livers, intestines, stomachs, tracheae, and esophagi. Eighteen heads were x-rayed using a standardized technique. Decolorization of the incisors, similar to that seen in fluorosis, anemia, vitamin A and vitamin E deficiencies, was observed, and fluorine as a causative factor was included. The decolorization was almost universal in the experimental groups; it was apparent in lesser degree in the pantothenic acid deficient animals, but was absent in the control series. Gross observations disclosed the presence of ulcers on the tongue, cheeks, and palate, as well as a change in the “furring” of the tongue. Soft deposits consisting of cell debris and bacteria accumulated on the teeth. The pathologic changes in the tongue and mucous membranes of the cheeks consisted of hyperplasia of the keratin layer and necrosis with ulcer formation. The latter had no typical location, and the hyperkeratinization appeared separate from or bordering on,the ulcers. The necrosis was granular in appearance and in the early stages was strikingly free of the usual inflammatory reaction. Histologic examination of the jaws and teeth showed a similar necrosis. Globular calcification of the teeth and extensive destruction of the interdental papillae and of the periodontal structures were seen. Gross alterations were discernible in the adrenal glands. Both the cortex and medulla showed large areas of hemorrhage and necrosis. These changes were noted in the experimental series and in the complete pantothenic acid deficient group.