An implantable atrial defibrillator applies cardioverting electrical energy to the atria of a human heart in need of cardioversion and thereafter paces the heart in a demand mode. The atrial defibrillator includes a first detector for detecting atrial activity of the heart, an atrial fibrillation detector responsive to the atrial activity detected by the first detector for determining when the atria of the heart are in need of cardioversion, and a cardiovertor responsive to the atrial fibrillation detector for applying the cardioverting electrical energy to the atria of the heart when the atria are in need of cardioversion. The defibrillator further includes a pacer for pacing the heart in a demand mode, a depletable power source for providing electrical power to the first detector, the atrial fibrillation detector, the cardiovertor, and the pacer, and an enable/disable stage for enabling the pacer in response to the cardiovertor applying the cardioverting electrical energy to the atria and thereafter disabling the pacer in response to the occurrence of a predetermined event for conserving the depletable power source.