This paper describes an experiment technique for the measurement of thermal conductivity, thermal diffusivity and perfusion using self-heated thermistors. Thermal probes are constructed by placing a miniature thermistor at the tip of a plastic catheter. The volume of tissue over which the measurement occurs depends on the surface area of contact between the thermistor and the tissue. Electrical power is delivered to a spherical thermistor positioned invasively within the tissue of interest. The electrical power and resulting temperature rise are measured by a microcomputer-based instrument. When the tissue is perfused by blood, the thermistor heat is removed both by conduction and by heat transfer due to blood flow near the probe. In vivo, the instrument measures effective thermal properties which are the combination of conductive and convective heat transfer. The accuracy of the conductivity and diffusivity measurements was evaluated by operation of the probe in media of known thermal properties. Perfusion measurements in canine liver, prostate, and spleen are presented.