An implantable heart stimulator comprising a right-atrial sensing unit, which has an input that is connected or is to be connected to a right-atrial electrode line, and which is configured to evaluate an electric input signal present at the input thereof in such a way that the right-atrial sensing unit detects at least one signal feature typical for a contraction of a right atrium and generates a corresponding output signal, further comprising a right-ventricular sensing unit, which has an input that is connected or is to be connected to a right-ventricular electrode line, and which is configured to evaluate an electric input signal present at the input thereof in such a way that the right-ventricular sensing unit detects at least one signal feature typical for a contraction of a right ventricle and generates a corresponding output signal, further comprising at least one right-ventricular stimulation unit, which has an output that is connected or is to be connected to a right-ventricular electrode line, and which is configured, in response to a control signal, to generate a right-ventricular stimulation pulse and to deliver this via the output, further comprising at least one left-ventricular sensing unit, which has an input that is connected or is to be connected to a left-ventricular electrode line, and which is configured to evaluate an electric input signal present at the input thereof in such a way that the left-ventricular sensing unit detects at least one signal feature typical for a contraction of a left ventricle and generates a corresponding output signal, further comprising at least one left-ventricular stimulation unit, which has an output that is connected or is to be connected to a left-ventricular electrode line, and which is configured to generate, in response to a control signal, a left-ventricular stimulation pulse and to deliver this via the output, and also comprising a stimulation control unit, which is connected to the right-ventricular sensing unit, the left-ventricular sensing unit, the right-ventricular stimulation unit and the left-ventricular stimulation unit and is configured to process output signals of the sensing units (58, 66) and to generate a control signal for the stimulation units (56, 64), wherein the stimulation control unit is configured to switch over its operating mode reciprocally between at least one right-ventricular stimulation mode, in which no control signals triggering left-ventricular stimulation pulses are output to the left-ventricular stimulation unit, and a biventricular stimulation mode, wherein the stimulation control unit is also configured to perform a switchover depending on the duration of a current QRS signal portion of a signal indicating a certain one or any of the electric input signals, characterised in that the stimulation control unit is configured to switch into the biventricular stimulation mode when a comparison of the signal indicating the duration of a current QRS signal portion with a first comparison value reveals that the duration of the current QRS signal portions is longer than a first reference value represented by the first comparison value, and switches into the right-ventricular stimulation mode when a comparison of the signal indicating the duration of a current QRS signal portion with a second comparison value reveals that the duration of the current QRS signal portion is shorter than a second reference value represented by the second comparison value, wherein the stimulation control unit is configured to determine the signal indicating the duration of a QRS signal portion such that the signal indicating the duration of a QRS signal portion reflects the time interval between contraction of the right atrium detected by the right-atrial sensing unit and a contraction of the left ventricle detected by the left-ventricular sensing unit and to be assigned causally to the right-atrial contraction.