The purpose of this study was to evaluate bone activity at the midpalatal suture that had been biomechanically stimulated by rapid maxillary expansion (RME). A rigid acrylic-bonded expansion device with a maxiskeleton screw was used for RME in three patients who were in three different growth periods. The screw was activated twice a day by 1/4 turns. After sufficient expansion, the screw was fixed with acrylic resin, and the same appliance was used as a removable retention plate for three months. Single photone emission computed tomography (SPECT) bone scintigraphy records were obtained before (T1) and at the end of RME (T2) and three months after the retention period (T3). According to the scintigraphic records, the increase in bone activity was highest in the anterior and medial sections on both the left and right sides of the maxilla in all cases. After three months of retention, the bone activity returned to its original level. Therefore, it may be stated that the retention period of three months was sufficient for bone reorganization.