The purpose of the present experiments was to study the effect of a rise in local venous pressure pressure (Pv) on interstitial fluid hydrostatic (Pi) and colloid osmotic pressure (COPi) in rats. The Pv of the hind limb was increased by ligating the iliac veins and the inferior caval vein. Interstitial fluid for protein and COP determination was sampled from subcutaneous tissue of the hind limbs by a wick technique, and the Pi was measured by a modified Scholander technique. When the Pv was increased from 3 to 12 mm Hg, the COPi fell from 9 to 4 mm Hg. The Pi averaged -1 mm Hg under control conditions and was unchanged when the Pv was elevated to 12 mm Hg. No edema was detected. Higher rises in venous pressures resulted in further but less pronounced falls in interstitial fluid COP. Edema developed and the Pi increased to +1 to 2mm Hg. The results show that the fall in COPi is an important edema-preventing factor during increased venous pressure, whereas a rise in Pi only seems to oppose further increase of an already existing edema.