Abstract: A method and apparatus for measuring the flow rates of fluids is disclosed. The apparatus comprises a probe which may be placed in a fluid inside a vessel, such as a blood vessel. The probe comprises at least one pair of conductors to which the circuit provides a small current. As ions are disordered or migrate between at least one of the conductors and the fluid, a current or charge is caused to flow between the conductors. The circuit measures this current using a differential amplifier and generates an output signal corresponding thereto. The output signal is then displayed on a monitor. Because the current flow is closely related to the rate of flow of the fluid past the probe, an accurate measurement of the flow rate is obtained.