A measuring instrument for obtaining an intracardial signal indicative of blood oxygen saturation, which may be used to control the stimulation frequency of a heart pacemaker, has a measuring probe which contains a light transmitter and a light receiver. The light receiver receives light from the light transmitter reflected by the blood. The light transmitter and the light receiver are connected in parallel to an evaluation circuit via two leads. A constant current source is connected in series with the light transmitter. The current generated by the light receiver, indicative of blood oxygen saturation, can be identified by forming the difference between current flowing in the measuring probe and the constant current. A reference measurement to compensate for factors in the total current resulting from the lead resistances and the measuring probe temperature is not necessary.