As previously reported, young rats on zinc-deficient diets develop a marked epithelial hyperplasia in the mucosa of the cheek, associated with a four-fold increase in the number of mast cells in the lamina propria. We now report the results of a quantitative and qualitative ultrastructural study of these mast cells. In contrast to the cells in mast cell-related disease, the cells were of normal morphology, but of consistently increased size, containing proportionally increased numbers of mitochondria and specific granules. The size distribution of the granules was as in controls. Microvilli were lengthened in proportion to the increase in the cell circumference. The number of nuclear pores was increased but their spacing was as in controls. In all these measurements, the experimental mast cells were a remarkably homogeneous population. We conclude that mast cells are capable of developing into cells of larger size showing evidence of greater activity. It may be that this development occurs in response to the same (unknown) stimulus which causes their number to increase. By virtue of their increased number and size, mast cells may play an important role in the enhancement of the microcirculation commonly seen in conditions of epithelial hyperplasia.