Purpose: IK_ACh blockade was expected to be a new strategy for treating atrial fibrillation (AF). However, the clinical utility is limited despite its atrial-selective actions. We analyzed electrophysiological effects of tertiapin on the rat dilated atria caused by chronic volume overload. Method: Four weeks after the surgery of aorto-venocaval shunt (AVS), electrophysiological testing was performed with isolated atria in the presence of carbachol. Results: Tertiapin at 30 and 100 nM prolonged the effective refractory period, but decreased the conduction velocity both in the atria of the sham-operated and AVS rats. The extents of the electrophysiological responses to tertiapin in the atria of the AVS rat were greater than those in the sham-operated ones. The low concentration of tertiapin increased the duration and cycle length of burst pacing-induced AF in the atria of AVS rat, showing proarrhythmic actions, whereas it shortened them in the sham-operated ones. Meanwhile, the excitable gaps in the atria of AVS rat were greater than those in the sham-operated ones. The high concentration of tertiapin attenuated the AF induction both in the atria of the sham-operated and AVS rats. Conclusion: Proarrhythmic properties of tertiapin can be manifest in the pathophysiologically modified atria caused by chronic volume overload.