By using a quantitative Evans blue technique, vascular permeability of Walker carcinosarcomas implanted in the liver was compared with that of the host liver tissue. Rats were sacrificed at intervals from one minute to 96 hours after the intravenous injection of the dye. Permeability activity curves were significantly different for tumors and normal liver. Permeability activity was lower in the tumor at one and five minutes, approximately the same in the tumor and liver from 15 minutes to one hour but, from two hours to 48 hours, the permeability activity was significantly higher in the tumor. The peak tumor-liver permeability activity ratio of 1.87 occurred at six hours, and the highest tumor permeability activity values were seen at 18 hours. Liver permeability activity, in contrast, increased gradually after dye injection, reading peak levels at 24 to 72 hours. At 96 hours, both tumor and liver permeability activity were lower and equal. The differences in permeability activity may be explainable on the basis of structural differences in normal and tumor vessels or may be due to chemical substances released as a result of the presence of the tumor. The differences in permeability activity of tumors and normal tissues may be an important factor in determining the effectiveness of diagnostic and therapeutic techniques.