BACKGROUND Pathological displacement of teeth caused by periodontitis-related bone loss in patients with diabetes is often corrected with orthodontic treatments. However, recovery from orthodontic therapy is often delayed for unclear reasons. This study explored effects of streptozotocin-induced diabetes in rats on protein expression involved in remodeling of the periodontal ligament (PDL) and alveolar bone during orthodontic tooth movement. METHODS Forty-eight Sprague-Dawley rats were randomly divided into two experimental groups: "normal" and "diabetes" (n = 24 each). Diabetes was induced by a single dose of streptozotocin (65 mg/kg). Animals were euthanized at 3, 7, and 14 days after orthodontic induction. Changes in expression of collagen type I (Col-I), matrix metalloproteinase type 1 (MMP-1), and tissue inhibitor of MMP-1 (TIMP-1) were measured immunohistochemically in the pressure side. Col-I and collagen type III (Col-III) fibers were assessed by picrosirius red staining in the tension side. Osteoclasts were observed on the surface of the alveolar bone. RESULTS Diabetes increased expression of MMP-1 and Col-III and decreased expression of Col-I in PDL. After the orthodontic induction, osteoclast action was delayed, and higher Col-III/Col-I and MMP-1/TIMP-1 ratios persisted in the diabetes group compared with the normal group. The ratio of MMP-1/TIMP-1 in the diabetes group reached a peak on Day 7, whereas the ratio remained at near control levels in the normal group. The diabetes group appeared to have worse recovery from damage caused by orthodontic movement. CONCLUSIONS Under mechanical forces, diabetes prolonged duration of degradation of PDL and remodeling of PDL and resorption of alveolar bone.