In order to monitor perfusion during certain surgical operations a simple instrument for measuring the rate of flow of a liquid in the range 2–20 ml/min was developed and has proved useful in the laboratory. A miniature thermistor is used as the measuring probe and a pulse technique to reduce heat dissipation in the liquid. The circuit described is self‐oscillating and its frequency is proportional to the liquid flow. A frequency‐to‐voltage converter allows a recorder to be driven directly but the response is not linear, the sensitivity being much higher for the lower rates of flow (i.e., recorder scale expanded at the lower end). The dimensions of the measuring cell and methods of temperature compensation are discussed. Calibration curves are shown as well as a typical flow recording. The same circuit may be adapted to measure flow in a gas.