The heat-induced environmental changes in tumor tissues are considered to influence the antitumor effect of hyperthermia or hyperthermochemotherapy, which is believed to complement the direct lethal effect of heat on tumor cells. The effects of local hyperthermia on the blood flow, oxygen pressure and pH in tissues were investigated using AH-100B tumor bearing rats, by immersing the tumor in a water bath at 41°, 43° and 45°C. These parameters were measured in the marginal and deeper sites of the tumor mass, and in the normal muscle adjacent to the tumor. During immersion at 41°C, blood flow in the tissue was increased at each site, and during immersion at 43°C, tissue blood flow increased initially at each site, but decreased with time to rates below that of the unheated tissue. During immersion at 45°C, the blood flow decreased markedly in each tissue. The changes in oxygen pressure and pH in each tissue were similar to those observed in the blood flow during localized heating at 41°, 43° and 45°C. In local thermochemotherapy, the initial stage of hyperthermic treatment seems to be the most suitable time for administering carcinostatics, since it is the time when tumor blood flow has not yet decreased.