The goal of this study was to investigate the heat sensitivity of the microcirculation in normal C3H murine leg muscle and a variety of transplanted tumour lines (KHT, SCC-VII, RIF-1, C3H mouse mammary carcinoma, two human mammary carcinomas MDA-468 and S5). Clearance rate of a radioactive tracer monitored following an intra-tissue injection was used as a measurement of microvascular integrity during heat treatment. Clearance rate in all tumours studied was significantly lower after 1 h of heating at 44 degrees C than the initial pretreatment clearance rate. Response of normal muscle differed from that of tumours in that the clearance rate after 1 h of heating at 44 degrees C was similar to the initial clearance rate. Vasculature in the KHT fibrosarcoma was more sensitive to heat treatment than that in other tumours. In response to a heat treatment at 43, 44, 45 and 46 degrees C the same level of microvascular damage occurred in half the time in KHT fibrosarcoma than in normal muscle. Furthermore, vascular damage in both muscle and KHT tumour followed the same relative isoeffect relationship, a 1 degree C change in temperature was equivalent to a change in heating time by a factor of two.