The plethysmanometer described follows the principle of the relaxed wall of the vessel. Here the pressure is adjusted in a finger cuff and by means of a pneumatic regulation is varied in such a way that the light plethysmogram of the adequate finger does not show any more pulsations. Thus the pressure effected on the finger above the cuff corresponds to the pressure in the finger arteries and is to be registered in mm Hg. When the course is measured in the same test person with changing of the arterial pressure (e.g. absolute arrhythmia or compressed-air pressure experiment) the blood pressure of the aorta measured when a heart catheter is diagnostically indicated shows a very close correlation to the unbloodily registered pressure of the finger arteries. On the other hand, greater varieties of the correlation coefficient which can be proved from test person to test person at present do not yet allow a formula obligatory in general, in order to tansfer the value unbloodily measured into the adequate bloody one. Nevertheless, already nowadays great ranges of application are possible for the progressing measuring of the blood pressure.