To investigate the impact of synovial fluid volume on oxygen tension (PO2) and other metabolic correlates, 24 specimens of synovial fluid from the knees of 22 patients were analyzed for volume, number of leukocytes (WBC), pH, PO2, PCO2, glucose, protein, and complement (CH50) levels. Concurrent arterial blood samples were obtained in 21 instances. Synovial fluid PO2 values varied inversely with volumes of synovial fluid (r = -0.54, P less than 0.01), but when patients with rheumatoid arthritis were excluded, the correlation was more significant (r = -0.76, P less than 0.001). When synovial fluid PO2 dropped below 45 mm Hg, intraarticular acidosis resulted. The decrease in pH (r = 0.93, P less than 0.001), the lowering of glucose values (r = 0.89, P less than 0.001), and the rise in PCO2 (r = -0.79, P less than 0.01) can be explained by a shift toward anaerobic metabolism coupled with the impaired elimination of its products. Systemic acidosis and hypoxia were not found. Intraarticular hypoxia most likely represents circulatory imbalance at the level of the synovial membrane, although an inverse relationship of synovial fluid PO2 and WBC was also noted. Complement and protein levels had no correlation with volume, pH, or respiratory gas tensions of synovial fluids. Our data support the importance of the effective blood flow to the joint in maintaining homeostasis. The volume of synovial effusion and the compliance of the joint capsule appear to be important determinants of the articular blood supply.