Abstract: A pacemaker for stimulating both the atrium and the chamber of a heart is able to inhibit the emission of chamber stimulating pulses if a spontaneous chamber reaction is sensed before the expiry of an AV interval, counting from the emission of an atrial stimulating pulse. To prevent the pacemaker from being locked in a position where consecutive chamber stimulating pulses are emitted because of retrograde atrial reactions making the atrium insensitive to atrial stimulating pulses, the AV interval is shortened after the emission of a chamber stimulating pulse. After a predetermined time, a predetermined number of pulses, or when a spontaneous chamber reaction is sensed within the shortened AV interval, the longer AV interval is restored.