Aims: To investigate the presence of endogenous b-endorphin, an opioid, in the synovial lavage fluid of the temporomandibular joint (TMJ), and to compare the concentration of ß-endorphin in patients with closed lock with that in symptom-free subjects. Methods: Thirty-eight patients (38 joints) with closed lock diagnosed on the basis of the results of clinical examination and magnetic resonance imaging (MRI) and 11 healthy volunteers (19 joints) were examined. Samples of lavage fluid were obtained prior to arthrocentesis by washing the joint with saline. Samples were assayed for b-endorphin by an enzyme immunoassay, and concentrations of protein were measured by a bicinchoninic acid assay. Subjective pain was assessed by patients using a visual analog scale. Bone changes in the condyle were assessed by MRI, and synovitis was assessed on the basis of arthroscopic findings. Results: ß-endorphin was present in the synovial fluid of the TMJ, and the concentration was significantly higher in patients with closed lock of the TMJ compared to symptom-free volunteers. The b-endorphin levels were not, however, significantly correlated with clinical parameters in the patients. Conclusion: The study results support recent findings that some opioids and their receptors exist not only within the central nervous system but also in the TMJ region, and that opioid concentrations are higher in patients with pain and dysfunction of the TMJ. Keywords: arthrocentesis, ß-endorphin, synovial fluid, temporomandibular joint