To test whether the pressure-volume relationship of the canine small intestinal vascula-ture is linear over the normal range of portal venous pressures (5 to 35 nun Hg), we used two methods to measure volume: (1) the integral of inflow minus outflow (IFD), and (2) tissue activity of5lCr-labeled erythrocytes (Cr-51). Venous pressures were changed in steps of 5 mm Hg with a servo-controlled system. The tissue was perfused at a constant rate. With venous pressures of 3.6 to 38.6 mm Hg, the vascular compliance was 2.19 ± 0.42 (SD) ml/kg − mm Hg using IFD, and 1.87 ± 0.50 ml/kg-mm Hg using Cr-51. Although a quadratic term significantly improved the fit, the effect was small (less than 3 ml/kg with a 30 nun Hg venous pressure change). The control blood volume of the intestinal loop at a venous pressure of 8.6 ± 1.5 (SD) nun Hg was 86.2 ± 19.1 ml/kg tissue weight using the mean transit time of a step input of indocyanine green at a perfusion pressure of 106 ± 29 nun Hg and a flow of 556 ± 147 ml/ruin − kg. We conclude that there is no significant change in compliance over the normal venous pressure range of 5-35 nun Hg.Circ Res 47: 551-558, 1980