The influence of tubal dysfunction on the osseous structures of the temporal bone of the guinea pig was studied in nondecalcified acrylic embedded tissue. There was a new formation of bone in stages depending on the duration and intensity of the controlled reduced pressure in the bulla which was checked by tympanometry. We found new bone in the Lamina propria of the mucous membrane. Adjacent osteoblasts produced newly formed trabecular bone, whose structure depended on load. A balance between new formation and absorption of bone resulted from release of the reduced pressure in the bulla. Normal pressure in the bulla was followed by an increased activity of the osteoclasts, but the newly formed bone was never resorbed completely. New phases of dysfunction of the Eustachian tube produced sclerotic zones of ossification in the original bone shaped like onion rings. This osteoneogenesis was not seen in the control group with normal impedance, and in the ears subjected to myringotomy, where operatively induced tubal dysfunction could not produce a reduced pressure in the bulla. The mechanism of new bone formation and absorption depends on structural differences. Its mechanism can be interpreted in the light of Frost's concept of remodelling of bone. These results confirm Wittmaack's theory, that pneumatization is determined by environmental factors.