abstract–From a material of 55 rats in which one maxillary molar had been moved buccally with a fixed orthodontic appliance, four specimens showing crystal‐like structures in pressure zones in the periodontal ligament were studied with the electron microscope. Crystals which occurred within the lumen of enlarged blood vessels appeared to have formed by direct transformation of erythrocytes. Crystals were also seen in extravascular locations. These particles seemed to have formed by solidification outside the blood vessels of substances released from degraded red blood cells. Many particles revealed a highly ordered internal structure thought to reflect the unit cell arrangement of the crystalline substance. On the basis of their location, internal structure, and the experimental circumstances, it was concluded that the crystals represented hemoglobin, rather than hematoidin, ferritin or hemosiderin. The study indicated that under the special conditions of pressure and hemostasis that existed in the periodontal ligament as a result of the experimental procedures, local degradation of erythrocytes occurred in a few specimens by mechanisms different from the physiologic breakdown of red blood cells in the spleen, liver and bone mar