Access to interstitial fluid is of fundamental importance to understand tumor transcapillary fluid balance, including the distribution of probes and therapeutic agents. Tumors were induced by gavage of 9,10-dimethyl-1,2-benzanthracene to rats, and fluid was isolated after anesthesia by exposing tissue to consecutive centrifugations from 27 to 6,800 g. The observed (51)Cr-EDTA (extracellular tracer) tissue fluid-to-plasma ratio obtained from whole tumor or from superficial tumor tissue by centrifugation at 27-424 g was not significantly different from 1.0 (0.92-0.99), suggesting an extracellular origin only. However, fluid collected from excised central tumor parts had a significantly lower ratio (0.66-0.77) for all imposed G forces, suggesting dilution by fluid deriving from a space unavailable for (51)Cr-EDTA. The colloid osmotic pressure in tumor fluid was generally higher than in fluid isolated from the subcutis, attributable to less selective capillaries and impaired lymphatic drainage in tumors. HPLC analysis of tumor fluid showed that low-molecular-weight macromolecules not present in arterial plasma were present in tumor fluid obtained by centrifugation and in venous blood draining the tumor, most likely representing proteins derived from tumor cells. We conclude that low-speed centrifugation may be a simple and reliable method to isolate interstitial fluid from tumors.