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Background Pacing-induced atrial electrical remodeling AER is characterized by shortening of atrial effective refractory period A-ERP and its altered rate adaptation. In paroxysmal atrial fibrillation AF , periods of AF occur with interveneing normal sinus rhythm NSR when atria recover from the preceding AER. Previous episodes of AF may precondition the atrial myocardium and cause different time course of AER in subsequent episodes of AF. But the influence of the preceding AER on the subsequent AER has not been described. Methods Four mongrel dogs were anesthetized with enflurane. After thoracotomy, silicon band with 3 pairs of electrodes was sutured to the lateral wall of the left atrium. Atrial pacing was performed after 2 wks of recovery and autonomic blockade. Pacing protocol consisted of rapid atrial pacing RAP at 500 bpm for 60 min and recovery in NSR for 60 min which was repeated three times. A-ERP was measured every 10 min. The same pacing protocol was repeated after pretreatment with verapamil 0.1 mg/kg/hr). Results 1 With 60 min of RAP, A-ERP decreased significantly 126 6 ms vs. 105 7 ms, p 0.005 . 2 After cessation of pacing, A-ERP returned to 98% of baseline value in 15 minutes. Recovery from AER occurred faster than AER 78 vs 21 ms/h . 3 After pretreatment with verapamil, RAP decreased A-ERP from 127 5 ms to 116 5 ms. AER, the reduction in A-ERP, was significantly attenuated by pretreatment with verapamil ERP 17 7 vs. 9 0.2 %, p 0.05 . 4 When RAPs were repeated, AER showed a tendency of acceleration, but it was not statistically significant ERP 22 ms, 24 ms, 28 ms at the end of 60 min pacing for the 1