The mechanism of pathologic bone resorption associated with cholesteatoma is controversial. It is now evident that transmitted pressure is a significant factor in the activation of osteoclasts. Previous studies on the effect of pressure have been confounded by the possible foreign body effect of materials placed into the middle ear to induce the pressure. Positive air pressures were maintained in the middle ear of Mongolian gerbils via implanted Silastic catheters. Histologic evaluation of the ventral bulla wall was performed after pressure had been applied for two weeks. A significant increase in the number density, area density, and size of osteoclasts present in the pressurized bullae was noted at pressures of 4, 6, 10, and 20 mm Hg, but not at 2 mm Hg. We conclude that minute increases in pressure, in the absence of foreign objects, stimulate osteoclastic bone resorption in the middle ear of the gerbil.