Objectives: The purpose of this study is to assess the effects of rapid maxillary expansion (RME) on metabolic activity in the temporo mandibular joints (TMJs) of young adult patients using scintigraphy. Methods: The images of the TMJs were obtained from the retrospective scintigraphic images taken from 17 adult females (between16.1 and 18.8 years of age, mean age of 17.3 ± 0.86 years) who had non-functional bilateral posterior crossbite, deep palatal vault, anddental crowding, and had been treated with RME. Bone scintigraphs were collected at 3 time intervals: at the beginning of treatment(T1), during the opening of the mid-palatal suture (T2), and at the end of screw activation (T3). Alteration in bone activity in the TMJregions were evaluated in sagittal and transaxial slices. To determine the differences between the intervals, repeated analysis of vari ance and Bonferroni multiple comparison tests were applied. Results: In the right and left TMJ regions, significantly increased metabolic activity was exhibited between T1 and T2 (P < .001). Atthe time of opening the maxillary mid-palatal suture, the metabolic activity increased by approximately 60% compared to the initialstatus. At the end of the active expansion period (T3), the change in metabolic activity was approximately 20% lower compared to T2. Conclusions: Metabolic activity intensified in the regions of interest in the TMJ during RME. After mid-palatal suture opening, theactivity noticeably decreased (T2-T3). This decrease in bone activity suggests that the TMJ complex adapts to RME forces.