The effects of hyperthermia on blood flow and intravascular volume were studied in mammary adenocarcinoma 13762A growing subcutaneously in the leg of Fischer F344 rats. The blood flow was determined using microspheres labelled with 125I, and the blood volume was determined using red blood cells labelled with 51Cr. At the end of heating with water bath at 43.5 degrees C for 1 hour, there was a marked elevation of 51Cr in tumor. The 125I content in tumor also was mildly elevated. Histologically there was a greater number of patent blood vessels per unit area, and they were dilated and hyperemic. In addition, widespread and diffuse hemorrhage could be seen. It appeared, therefore, that the increased 51Cr and 125I label in the tumors immediately after heating was, at least in part, a result of leakage of the labels to extravascular space in addition to possible vasodilation and increased blood flow. At 1 and 5 hours after heating, tumor blood flow was considerably reduced, and at 16 hours both tumor blood flow and blood volume were considerably reduced. Histological examination demonstrated that the tumor blood vessels remained dilated and hyperemic after heating. The effect of heat on blood flow and blood volume in the skin and muscle adjacent to the tumors was also investigated. Blood flow and blood volume in the surrounding normal tissues were significantly elevated at the end of heating. Blood flow was relatively unchanged at 1, 5, and 16 hours after heating, but blood volume was reduced to about one half. These findings indicate that hyperthermia may induce greater damage to vasculature of tumors than normal tissues, and that vascular damage in tumors may take some time to express itself following moderate hyperthermia.