We are studying the biologic aspects of vertical movement of teeth, which are often used in orthodontic treatment involving variations in alveolar tissue. In the present study, the four upper incisors of five infant Macaca fuscata monkeys were intruded vertically from 1.1 to 5.5 mm. The following effects were examined: (1) movement of the gingiva, (2) change in the depth of the gingival sulcus, and (3) microscopic effects on the alveolar tissue. The results were as follows. (1) The gingiva moved in the same direction that the teeth were intruded, but only about 60% as far. (2) The clinical crown shortened and the gingival sulcus deepened. The shortening of the crown and the deepening of the sulcus were both approximately 40% as much as the tooth intrusion. (3) There was no inflammation or swelling microscopically in the gingiva of either the experimental animals or the controls. (4) The epithelium was always attached in the cementoenamel junction, even when the tooth was intruded. As the tooth intrusion was increased, the dentoperiosteal fiber (DPF) and the dentogingival fiber (DGF) terminating in the cementum gradually parted from it; when the tooth was intruded more than 5.0 mm, few fibers terminated in the cementum. It was concluded that the gingival sulcus deepened with horizontal tooth intrusion because of an accumulation of gingival tissue applied with good oral hygiene--not because of swelling around the gingival margin or apical movement of the gingival pocket--and the DPF and the DGF were parted from the cementum gradually as the tooth intrusion increased.