A biventricular cardiac stimulator has a right-ventricular sensing unit, having or connected to a terminal for a right-ventricular sensing electrode, a left-ventricular sensing unit, having or connected to a terminal for a left-ventricular sensing electrode, and a pacemaker timer, which is connected to the right-ventricular sensing unit and the left-ventricular sensing unit. The cardiac stimulator has a programmable automatic switch, which is effectively connected to the pacemaker timer to switch the pacemaker timer optionally between a primarily right-ventricular control and a primarily left-ventricular control, as well as an evaluation unit, which is effectively connected to the switch and is designed to detect and evaluate at least one stability parameter that is characteristic of the stability of the electrode position of the left-ventricular sensing electrode, whereby the programmable automatic switch is designed to automatically switch the pacemaker timer control as a function of a value of the detected stability parameter.
