The morbid changes associated with deficiency of the antineuritic factor of the vitamin B complex (hereinafter designated vitamin B) are not well established. This is in large part due to the fact that the vitamin B complex has been only recently separated into its component parts, the earlier anatomical studies having been made of animals which had been given diets deficient in more than one factor. With the perfection of experimental diets the prospects of understanding more satisfactorily the structural derangements occasioned by a deficiency of vitamin B seem brighter. The present report is one result of such a study. It is concerned with the occurrence of ulcerations in the gastric and duodenal mucosa of rats whose diet had been deficient in vitamin B.