An implantable atrial defibrillator provides cardioverting electrical energy to the atria of a patient's heart in need of cardioversion and a precardioversion warning to the patient. The atrial defibrillator includes a detector for detecting atrial activity of the patient's heart, and an atrial fibrillation detector responsive to the atrial activity detector for determining when the atria of the patient's heart are in need of cardioversion. A cardioverter responsive to the atrial fibrillation detector, applies warning electrical energy to the atria of the patient's heart when the atria are in need of cardioversion and thereafter applies cardioverting electrical energy to the atria. The warning electrical energy is of insufficient quantity to intentionally cardiovert the atria but of sufficient quantity so as to be discernable by the patient.