BAS Medical is investigating the use of relaxin to improve ortho-dontic treatments. Relaxin is well known for its effects on remodeling soft tissue, and we believe relaxin will be clinically useful in speeding tooth movement and preventing relapse. We investigated the use of relaxin in preventing relapse in a dog model. Dog maxillary second incisors were orthodontically rotated an average of 42 degrees, and then relaxin was administered by gingival injection to relieve the rotational memory in the connective tissues. Teeth were retained for 30 days to allow fibers to reform. Teeth then were released and relapse was measured by a series of impressions. Animals receiving relaxin gingival injections (n = 8) were compared with placebo-treated animals (n = 8) (exhibiting high relapse) and gingival fiberotomies (n = 8) (low relapse). Gingival fiberotomy is a surgical procedure to cut the gingival connective tissues away from the tooth and has been shown to be effective in preventing relapse clinically and in animal models. There was a significant difference in relapse between the fiberotomy and the placebo control groups, and the relaxin-treated group had an intermediate response between the two groups (nonsignificant). Dose and treatment optimization may improve the response in future studies. To study the underlying mechanisms, we have localized relaxin receptors on gingival and periodontal ligament fibroblasts in tissue slices and cell cultures. Relaxin was found to stimulate collagenase production by relaxin in human gingival fibroblast cultures. Taken together, the data support a role for relaxin therapy to speed tooth movement and prevent relapse in orthodontic practice.