After normal tooth mobility was measured for thirty‐two incisors in six adult male monkeys (Macaca irus) trauma to the mesial or distal gingiva was produced by passing a scalpel blade down the gingival crevice until the alveolar crest was reached. Force was then applied towards or away from the lesion and the mobility reassessed. A fine steel blade was then passed into the periodontal membrane on the same side as the gingival lesion to a depth of the apical one third, a manoeuvre which was completed in one and a half minutes. The mobility was again estimated. Significant increases in mobility were produced when the periodontal tissues were traumatised but the increases in mobility which followed disruption of the gingival regions were significant only on the side of compression. Damage to areas of tension in the membrane caused an increase of similar magnitude to trauma of areas of compression. It is concluded that both these mechanisms operate when horizontal force is applied to teeth in the approximal direction and appear to be of equal importanc