Open-ended plastic tubes were used as capsules for obtaining interstitial fluid from rabbits. The capsule was a 2.5 cm long plastic tube with an inner diameter of 6 mm. Three small incisions were made in the dorsal mid-line of the anaesthetized rabbit; two capsules were inserted into each incision. A sample of capsular fluid was obtained 6 weeks later by inserting a hypodermic needle through the skin. The volume of fluid obtained from the capsule was sufficient for the analysis of total protein, albumin, albumin:globulin ratio, colloid osmotic pressure and the fluid viscosity. Despite the significantly lower total protein, albumin, globulin and colloid pressure of intracapsular fluid compared with plasma, the intracapsular fluid was found to have a greater viscosity. It is our opinion that the increased viscosity of the intracapsular fluid is due to the presence of a high molecular weight substance other than albumin and globulin, possibly hyaluronan.