The effects of hyperthermia on vascular permeability in Walker carcinosarcomas and host liver tissue were studied in Sprague-Dawley rats. A quantitative Evans blue technique was used to measure permeability. With tumors heated to 40 degrees C, a nontherapeutic level, no changes in tumor vascular permeability as compared to control levels were noted. However, with tumors heated to 43 degrees C, within the therapeutic range of hyperthermia, significant rises in tumor vascular permeability occurred. Permeability was increased at both time periods studied, 30 minutes and 6 hours after hyperthermia and injection of Evans blue. These changes are similar to those seen after physical damage from freeze-thaw. It is likely that alterations in tumor microcirculation play a role in the therapeutic effect of intense hyperthermia.