The hypothesis has been advanced that synovium offers the main resistance to fluid escape from joints, even though it is under 20 µm thick. To test this, fluid was infused into the knee joint cavity of anaesthetised rabbits to set up a pressure gradient, then the profile of periarticular interstitial fluid pressure (Pif) was measured by advancing a micropipette, connected to a servo-null pressure recorder, in steps through a periarticular tissue 'window' until the joint cavity was entered. With intra-articular pressure (Pj) raised to 15 cmH2O (the pressure of an acute joint effusion) the pressure gradient dPif /dx (where x is distance) across the synovial lining was 0.47 ± 0.04 cmH2O µm-1 (n = 10 joints). This was 23.5-fold greater than the gradient in the subsynovium (0.02 ± 0.01 cmH2O µm-1; P < 0.0001, Student's t test), indicating that the hydraulic resistivity of the subsynovium is <=<>4 % of that of the synovium. The pressure profile was not altered by circulatory arrest. To test the hypothesis further, the effect of a stab perforation of the synovial lining on fluid drainage rate ((·Qs) was studied. Perforation raised both ·Qs and the conductance term d ·Qs/dPj more than 10-fold (n = 6 joints; P < 0.0001, ANOVA). The results thus support the view that, despite its thinness, the synovial lining offers the main hydraulic resistance to fluid drainage from a synovial joint. Experimental Physiology (2001) 86.6, 739-747.