Automatic continuous non-invasive measurement of blood pressure using the Penaz' principle (Finapres) was compared with the blood pressure records using a sphygmomanometer during anaesthesia in 29 patients without significant arteriosclerosis. There were no significant differences between values recorded using the two methods. Furthermore, the changes recorded in systolic, diastolic and mean arterial pressures were not significantly different. Also, the pulse recorded with the Penaz' principle corresponded to the heart rate recorded from ECG. Automatic continuous non-invasive measurement of blood pressure using the Penaz' principle offers a valuable aid for monitoring patients during anaesthesia.