Abstract: A device for determining the depth of anaesthesia of a patient, the device including an optical measuring means (1) for measuring on a part (2) of the patient an optical parameter such as transmission or reflection, which is influenced by the circulation of the blood through this part of the patient's body, and to produce an electric signal which varies in accord with this parameter. This signal is related to the periodic variations in the blood-pressure of the patient and thus has a frequency which coincides with the heart frequency of the patient and a maximum amplitude and a minimum amplitude which are related respectively to the systolic and diastolic blood-pressure of the patient. The signal is applied to signal processing circuits (3) which determine the variation in the period of the signal (.DELTA.(R-R)) and the variation in its maximum amplitude (.DELTA.P.sub.max) or minimum amplitude (.DELTA.P.sub.min) or in the difference between its maximum and minimum amplitudes (.DELTA.(P.sub.max -P.sub.