Measurements of the local blood flow through organs by means of heated thermistor-probes enable one to perform continuous recordings of flow. The principle is based on the measurement between a heated and an unheated probe, both implanted in the tissue or placed on the superficial layer of the organ. The flow-measure for this type of blood flow recording is the heat transported from the heated site of the tissue. It is measured in cal-cm(-1)-sec(-1)-degree C(-1). This is the dimension of the heat conductance. Quantification of continuous blood flow measurements in ml flow/g tissue-min has been carried out with combinations of gas clearance and heat transport. It has now become possible to measure blood flow in ml blood-ml(-1) tissue-min(-1) solely with the aid implanted thermistors. Two types of local blood flow measurements are described. 1. Measurements with "slug heating" by which a thermistor is heated for a very short time and the temperature change is subsequently recorded. 2. The measurement of flow by sudden introduction of heat, if a heating coil around a micro-thermistor is switched on or off and the course of the tissue temperature is subsequently followed. It is proven, that the temperature of a point-shaped micro-thermistor in nonperfused tissue and in homogeneous perfused tissue differs only by an exponential factor (see article) where uslug is the temperature field after a slug injection of heat dependent on location, time, blood flow (phi) and partition coefficient for heat (lambda) and where index 0 = unperfused tissue. If the heater is not heated in the form of a delta function but is switched on and the temperature is subsequently recorded or if the heater, after a period of constant temperature, is switched off, the derivatives in respect to time differ only by an exponential factor and can be used as a measure of flow. The evaluation of blood flow is described in detail. The measurements show the following advantages, compared with former techniques: 1. Heat clearance and continuous blood flow measurements by means of heat transport measurements are both possible at the same site. 2. No other operation and procedures are necessary except implantation of the thermistor. 3. Turning on the heater does not affect the tissue and does not cause emotional reactions. As examples the measurements of local blood flow in brain tissue and in the myocardium of conscious animals are described.