1. The flow of Ringer solution or paraffin oil from an infusion reservoir into the cavity of the knee (stifle) joint was measured in anesthetized rabbits, as intraarticular pressure was progressively elevated from its intrinsic slightly subatmospheric value to +25 cm H2O. 2. Paraffin oil did not penetrate the tissues lining the joint cavity, yet a continuous flow of oil occurred into the joint at pressures over +2 cm H2O. It was concluded that the joint investment behaved as a visco-elastic tissue. 3. Trans-synovial flow of Ringer solution was calculated by correcting the observed inflow for visco-elastic expansion of the joint capsule. At intra-articular pressures +2 to +9 cm H2O, trans-synovial flow increased at an average rate of 0.49 microliter min-1.cm H2O-1. The hydraulic conductivity of the synovium was therefore similar to that of subcutaneous connective tissue. At around +9 cm H2O, the 'breaking pressure', the slope of the pressure-flow relationship increased by almost sixfold to 2.81 microliter min-1.cm H2O-1. 4. Changes in joint visco-elasticity, synovial surface area, blood pressure, colloid osmotic pressure of plasma and of joint fluid, and inflammation were excluded as explanations of the marked increase in rate of fluid absorption, which is tentatively attributed to increases in synovial hydraulic conductivity. Some physiological and clinical implications of the data are discussed.