Changes in capillary blood flow were determined by serial measurements of xenon-133 (133Xe) washout rates from "Slow-line" C3H-mouse mammary carcinomas transplanted onto the flank or hindfoot, from non-tumor-bearing hindfeet, and from the lumen of a short portion of the proximal jejunum. The core temperature of the pentobarbital-anesthetized mice was externally controlled. Special techniques allowed selective rapid and uniform heating by water bath immersion of even the flank tumors and nonexteriorized jejunum. Immersion in a 44 degrees C bath immediately doubled washout rates not only from the immersed tumor, but also from a contralateral tumor that was not heated. This result and behavioral observations were consistent with an arousal-induced increase in cardiac output to the tumors that increased the capillary blood flow in them. In contrast, washout rates from the hindfeet or intestinal lumen did not increase unless the foot or intestine in question was itself heated, which is consistent with normal tissues having intrinsic mechanisms for regulating their own capillary blood flow that the Slow-line tumor lacks. This lack could complicate the design and interpretation of hyperthermia experiments in vivo. Immersion of Slow-line tumors in a 44 degrees C bath for 20 min did no microvascular damage detectable from 133Xe washout rates during the heating or for 1 hr subsequently. Washout rates from both foot and flank tumors had dropped by one-half after 41-43 min in the 44 degrees C bath. Equivalent declines had occurred after 60 min for the intestine and 100 min for the hindfeet.