The steady-state response and operating characteristics of a new thermal sensor for surface measurements of local tissue perfusion have been analyzed theoretically and evaluated in vivo. The flow measurement system incorporates an electrically isolated thin-film thermal sensor, which is maintained at a fixed temperature by high frequency response electronic circuitry. The sensor rests on the tissue surface, and the power required to maintain a fixed probe to tissue temperature elevation is measured and related to tissue blood flow. A theoretical analysis of the steady-state probe response to flow changes was carried out employing the bio-heat-transfer equation and a solution based on Fourier series to describe the temperature distribution within the tissue domain. A comparison of steady-state theory to results obtained from initial experimental tests on the surface of the dog heart, over a perfusion range 0.51 to 2,00 ml/min/g, shows close agreement. The probe demonstrates good sensitivity to flow changes, provides stable and continuous measurements, and appears promising for both research and clinical applications.