The flow of blood in a vessel (G) in a living being (P) which is exposed to a magnetic field (B) is determined by measuring the potential difference (.DELTA.V) between at least two points (P1, P2) next to the vessel (G). The potential difference (.DELTA.V) is produced on the basis of the Hall effect by the moving charge carriers (Q) in the blood in the presence of the magnetic field (B). When recording an ECG, the cardiac output can be determined in this manner under the influence of a magnetic field (B). At the same time, the ECG signal can be corrected in the knowledge of the component (T) of the ECG signal (E) which can be attributed to the Hall effect.