Increased vascular permeability was studied in the synovium of the rat's stifle joint following mechanical trauma and haemarthrosis. The abnormal permeability was detected by injecting colloidal carbon intravenously and examining the synovial vessels for intra-mural deposits of carbon. The effects of rotational and stretch injuries were minimal except when accompanied by intra-synovial haemorrhage. Injection of autologous whole blood into the joint evoked a marked venular permeability response which persisted for 24 hours. These findings indicate that the major factor in the formation of traumatic joint effusions is haemorrhage into the articular cavity or the adjacent tissues.