The experiments were performed in 15 dogs fed a diet which allowed dental plaque accumulation. A phase of experimental periodontal breakdown was initiated on day 0. After 210 days five dogs were sacrificed. In the remaining 10 dogs the periodontal pockets around the fourth lower premolars (4P and P4) were eliminated. During surgery a notch was prepared in the root at the level of the marginal termination of the alveolar bone in order to facilitate measurements in histological sections. From day 210 to day 450 the teeth of the animals were brushed twice daily. After 60 days of healing, i.e. on day 270, five of the dogs were sacrificed. Trauma from occlusion of the jiggling type was on day 270 produced in the P4 region of the remaining dogs by the installation of a cap splint and a bar device. The final five dogs were sacrificed on day 450. Following sacrifice, tissue sections comprising 3P, 4P, 1M and P3, P4, M1 were produced and subjected to microscopic analysis. The experiments revealed that, in the dog, forces produced by occlusal trauma are unable to induce a phase of progressive destruction of the periodontal tissues in tooth regions where the supporting tissues are markedly reduced but non-inflammatory.