A recent model predicted a local synovial circulation of extravascular fluid in which capillary filtrate entered a joint cavity and fluid simultaneously drained out of the cavity via intercellular pathways distant from capillaries. Here, solutions of albumin (10-250 g l-1) were infused continuously into the knee joint cavity in fifteen anaesthetized rabbits at 3-18 cmH2O pressure and steady-state net trans-synovial flow was recorded. The intra-articular fluid collected after 15-30 min was substantially diluted despite a sustained net efflux from the cavity. This supports the prediction that net efflux comprises an influx of low-protein plasma ultrafiltrate and a larger efflux of joint fluid.