To elucidate the mechanism of enhancement of transport of anticancer drugs into tumor tissue due to angiotensin II (AII), changes in microvascular pressure (MVP) and interstitial fluid pressure (IFP) due to AII were measured in normal subcutis and subcutaneous tumor in rats. Two different tumor cell lines AH109A and AH272 were used. MVP and IFP were measured using a microocclusion method and a diffusion chamber method, respectively. Insignificant changes in the pressure of normal venous vessels and the IFP of normal subcutis were brought about by AII-induced hypertension. In both AH109A and AH272, the IFP of the same region within the tumor increased with enlargement of tumor volume. The IFP of AH109A tumor (tumor volume = 1.8-2.2 cm3) increased slightly due to AII, but the tumor MVP of AH109A and AH272 increased markedly. Though both tumor IFP and MVP increased significantly due to AII, the increases the hydrostatic pressure difference between tumor vascular and interstitial spaces and leads to enhancement of the efficiency of filtration from tumor vessels to tumor tissue, which is one of the mechanisms of enhancement of drug delivery to tumor tissue due to AII.