Tissue Blood Flow is measured by applying a combined procedure of two independent approaches based on heat clearance: the Pulse Decay Method and the Continuous Method. The Pulse Method allows absolute assessment of tissue BF with no need for calibration, and can be applied only if the tissue BF is steady during the period of measurement. On the other hand, the Continuous Method enables the observation of rapid changes in tissue BF, and can be applied under non steady-state conditions. Using the combined method, a continuous quantitative measurement of transient changes in tissue BF can be obtained. For this purpose, we have developed two experimental systems consisting of independent electronic units: a Pulse Unit and a Continuous Unit. A micro-computer with dedicated software controls the operation of the electronic units and calculates tissue BF on-line. In vitro measurements are performed and demonstrate the reliability of the methods. In vivo measurements in rat brain tissue are also performed and include physiological and pharmacological changes of local tissue BF. The results of the two heat clearance methods correlate well with tissue BF values measured by a third independent method, the Hydrogen Clearance Method.