The invention concerns a cardiac pacemaker comprising a stimulation pulse generator (RVP; LVP) for biventricular stimulation of a heart, which is to be connected to at least one right-ventricular electrode for the stimulation of a right ventricle of the heart and to at least one left-ventricular electrode for the stimulation of a left ventricle of the heart and is connected to a control unit and is adapted to trigger right-ventricular and left-ventricular stimulation pulses with an interventricular delay time which is adjustable by means of the control unit. The invention is characterized in that the control unit is connected to an impedance detection unit which is to be connected to intercardiac electrodes and is adapted to form from an input signal formed by the impedance detection unit and dependent on the intracardiac impedance, an output signal indicating an optimum biventricular stimulation mode.