Time-temperature relationships are of critical importance in hyperthermia effects on tumors. Knowledge of temperature distributions within tumors is necessary to achieve maximal tumor response, and because blood flow is the major determinant of heat dissipation within tissue, knowledge of blood flow parameters is equally important. A mathematical model has been developed which estimates a parameter that may be related to regional tissue blood flow and is readily adaptable to clinical observations. Eight canine mast cell tumors were heated with interstitial radiofrequency (100 mHz) hyperthermia to a temperature range of 44 degrees C, +/- 0.5 degrees C, for 40 minutes. Estimated blood flow increased over the 40-minute treatment interval from 81 ml/min/100 gm of tissue at 10 minutes post-initiation of treatment to a maximum of 101.2 ml/min/100 gm of tissue at 30 minutes post-initiation of treatment. Over a 9-day period, during which both tumor and normal tissues were treated four times, values increased within the volume of interest. Maximum estimated blood flow within normal tissue increased from 115.7 ml/min/100 gm of tissue after 10 minutes of heating to a maximum of 121.7 ml/min/100 gm of tissue at 40 minutes. In contrast to normal tissue, eight canine mast cell sarcomas showed little change in estimated blood flow during 40 minutes of treatment. However, tumor tissue appears to undergo compensatory changes over the 9-day treatment interval with increases occurring in blood flow over that time period. These data underscore the importance of knowing blood flow characteristics within tumor and normal tissue.