Abstract: To obtain optical one-way transmission of bioelectrical potential differences between electrodes placed on a patient and the evaluation apparatus, one of the electrodes placed on the patient is designated as the reference electrode. The potentials of all the other electrodes are referred to this electrode, and the signals derived from the potential differences so obtained are amplified, multiplexed, converted analog/digital and transmitted in coded form. At least one of the potential differences recorded by the evaluation apparatus is derived from the difference between two transmitted signals. The number of signals transmitted can thus be less than the number of potential differences recorded. This process is particularly useful for transmitting electrocardiogram signals (ECG signals) to an ECG apparatus which produces twelve standard derivatives. The optical transmission can also be performed by an optical fibre.