1. Twenty-six control and twenty-six experimental animals were studied. 2. In the control rabbits bony union occurred in four weeks; in the control dogs it occurred in six weeks. 3. When the animals received anticoagulants in the preoperative period, they showed evidence of delayed union. See Table in the PDF file 4. When the anticoagulants were given in the immediate postoperative period, fibrous union resulted. 5. When heparin or dicumarol were given one week after the operation, delayed union occurred. 6. It is not the authors' intention to conclude from this study that non-union will necessarily occur in a patient who has received anticoagulant therapy; they feel, however, that the possibility of its occurring must be considered. 7. The findings of this experimental study are of interest and may have a clinical significance and application. It is hoped that these findings will stimulate further interest in the general field of fibrous tissue behavior.