Nitric oxide is thought to be involved in the pathogenesis of temporomandibular joint internal derangement. The published papers in this field led us to develop this study which aim was to evaluate nitric oxide metabolites (nitrite and nitrate) in synovial fluid of patients with temporomandibular joint internal derangement, and correlate them with the main clinical and radiographic parameters of the disease. The indication of arthrocenthesis was the main criteria for the selection of patients. At the time of arthrocenthesis, synovial fluid was collected to evaluate nitrite and nitrate. A craniomandibular index and visual analog scales were used for clinical evaluation of the patients. Total (Newbauer chamber) and differential counts of the infiltrated cells were performed. Nitrite and nitrate were quantified by Griess reaction. 17 patients (14 females and three males), and 22 temporomandibular joints were analyzed, with mean age 32.40 ± 1.91 years, and mean disease duration of 61.54 ± 16.78 months. With reference to the type of temporomandibular disorder, one patient showed disc displacement with reduction, seven showed disc displacement with reduction and episodes of locking, six showed disc displacement without reduction, six presented osteoarthrosis, and two had rheumatoid arthritis. No inflammatory cells were detected in the synovial fluid. No difference was found between nitrite and nitrate levels in the studied groups. No correlation was found between nitrite and nitrate levels and the analyzed parameters: craniomandibular index, interincisal opening, age, duration of disease, and radiographic grade. The pain levels detected by a visual analog scale positively correlated with synovial nitrite and nitrate levels ( p < 0.05). The results revealed that nitric oxide metabolites are related with pain levels in internal derangement, osteoarthrosis, and rheumatoid arthritis of the temporomandibular joint.