Blood flow is an important parameter for obtaining uniform thermal distributions in tumour and normal tissue. This study investigated the effect of fractionated hyperthermia on muscle blood flow in 30 dogs treated interstitially. These animals were divided into five groups, each group receiving either 0 (control), 1, 2, 3, or 4 hyperthermia fractions separated by 72 h. Each animal was treated for 40 min at 45 degrees C. Blood flow was measured with four different radioactive microspheres at either 10, 20, 30, or 40 min of heating. During the first treatment, blood flow increased from control of 7.5 +/- 1.1 ml min-1/100 g of tissue to 39.6 +/- 5.8 ml min-1/100 g of tissue at 20 min of heating. Blood flow decreased over the next 20 min to 24.4 +/- 4.8 ml min-1/100 g of tissue. This pattern was repeated for all hyperthermia treatments and peak blood flows were observed for all groups between 20 and 30 min of heating. Peak blood flows reached 20.0, 16.5 and 11.0 ml min-1/100 g of tissue for animals treated with 2, 3, or 4 hyperthermia fractions, respectively. These data suggest that peak blood flow in normal tissue decreased with increasing numbers of hyperthermia fractions. Blood flow response to hyperthermia changes from fraction to fraction and description of the kinetics of these changes is important for understanding the response of normal tissue to heat.