Rats 16 weeks old were fed a liquified diet consisting of a suspension of finely ground commercial rat pellets for a period of 35 days. Weight-matched controls were fed isocaloric amounts of the dry pellets. Numbers of dividing epithelial cells and dimensions of the cellular and keratin layers were determined using precisely matched homologous areas in histologic sections from experimental animals and controls. Three mucosal regions which during the masticatory movements in controls are subjected to friction, compression and deformation ("masticatory mucosa") and one region subjected to distension and friction ("lining mucosa") showed experimental mitotic rates ranging from one-fourth to less than half the rates in controls. The epithelium of experimental masticatory mucosa showed consistent, statistically significant atrophic changes in both cellular and keratin layers, while that of lining mucosa showed atrophic changes in the cellular layer only. It is concluded that the concept of disuse atrophy should be extended to include oral surface epithelia.