Six weanling male Sprague Dawley rats were fed a diet containing 0.4 ppm Zn and seven were fed an identical diet except for 40 ppm Zn. After 4 weeks, specimens of buccal mucosa in the region facing the molar teeth were removed. Paraffin sections, cut at 6 micron, were stained with toluidine blue, and tracings made of five sections per animal, spaced no less than 60 micron apart. Counts of mast cells of five sections length of section were made in a superficial zone of the lamina propria of 50 micron width and a deeper zone of 250 micron width. The average number of mast cells, per mm in the subepithelial zone of the experimental animals was 15.4, the range 9.2-33.1. The control average was 4.0; the range was 2.9-5.3. No increase was found in the deeper zone. The epithelium was parakeratotic and its thickness was increased two-fold. In the peripheral portion of the section, cellular and keratin layers were evenly increased in thickness, but in the central portion a disproportionate, nearly four-fold increase occurred in the keratin layer and a lesser increase in the cellular layer.