A dual chamber heart pacemaker has atrial and ventricular pulse generators (21, 22) and sense amplifiers (25, 26) which connect respectively to an atrial terminal (11) and a ventricular terminal (12) for connection to the heart. In an atrial synchronous mode, the pacemaker provides ventricular stimulating pulses in response to detected spontaneous atrial contractions, and spaced from them by an A-V interval determined by a timer (31). In an atrial-ventricular sequential mode, a timer (40) causes the pacemaker to deliver sequential atrial and ventricular stimulation pulses which are separated in time by an A-V interval determined by another timer (30). The atrial-ventricular intervals for atrial synchronous mode and atrial-ventricular sequential mode are independent of each other, permitting selection of different A-V intervals for the two modes of operation.