The present experiments were designed to investigate the effects of microwave hyperthermia on the microcirculation of normal and malignant tissues in female Wistar/Furth rats. During the localized heating of anesthetized rats, the hind leg musculature surrounding the SMT-2A mammary adenocarcinoma was heated at either 39 degrees, 42 degrees, or 44 degrees. Neither the body temperature, cardiac output, heart rate, nor systemic arterial pressure were significantly altered by heating at these temperatures for up to 60 min. Our results demonstrate that the changes in vascular resistance which occur during hyperthermic treatment are dependent upon both the temperature and the tissue heated. When the tumor (1.2 g)-bearing hind leg was heated to 39 degrees, the tissue vascular resistance and blood flow were unaltered even after 45 min of heating. Heating at 42 degrees and 44 degrees caused an initial vasoconstriction in the tumor, which was subsequently followed by marked vasodilation. This transient initial decrease in blood flow was not, however, observed in the skeletal muscle at either temperature. With prolonged heating at 42 degrees and 44 degrees, the muscle blood flow increased by a factor of 1.6 and 3.2, respectively. In contrast, malignant tissue blood flow increased by a factor of 1.3, and this maximum increase was observed only when the tumor was heated at 44 degrees for more than 45 min. Nevertheless, even with this proportionally greater increase in the blood flow of the surrounding normal tissue, it was never more than that of the tumor. As a consequence, the tumor temperature during hyperthermic treatment was always either less than or equal to that in the surrounding normal musculature.