Abstract: Apparatus for measuring the flow of blood or other liquids through a channel includes a flexible catheter on the exterior wall of which are mounted a heating coil and spaced therefrom, a thermistor. The catheter is placed in a channel in which the liquid flow is to be measured so that the thermistor is positioned downstream of the heating coil. The apparatus also includes a capacitor which is charged to a certain level by a power supply source and then periodically discharged to the heating coil to cause the coil to produce heat. The heating coil thus heats liquid flowing thereby and any temperature change is detected by the thermistor and supplied to temperature measuring circuitry. This circuitry determines the temperature change in the liquid caused by the heating coil, integrates this temperature change and takes the reciprocal of the integrated temperature change, and produces therefrom a measure of the liquid flow.