A single-chamber heart stimulator has an electrically conductive housing, first and second detection units, and a VT/SVT discrimination unit. The first detection unit is connected to an electrode pair of a ventricular electrode lead for obtaining a first electrocardiogram signal recorded in bipolar fashion, and is designed to sense and classify ventricular heartbeats. The second detection unit is connected on one side to an electrode of the ventricular electrode lead, and on the other side to the electrically conductive housing, for obtaining a second electrocardiogram signal containing signal features characteristic of atrial heartbeats. The VT/SVT discrimination unit is connected to the first and second detection units, and effects discrimination between ventricular and supraventricular tachycardias based on ventricular heartbeats measured through the first detection unit, and on atrial heartbeats measured through the second detection unit to the extent the rate of measured ventricular heartbeats exceeds a threshold value (VT zone threshold).