While class Ic antiarrhythmic drugs are effective in treating patients with atrial fibrillation, their mechanism of action is poorly understood. We performed electrophysiological studies before and after their administration to 22 patients with paroxysmal atrial fibrillation. Atrial refractoriness, maximal interatrial conduction delay and the wavelength index were measured at two basic cycle lengths (600 or 400 ms). Both drugs studied increased atrial refractoriness and wavelength index. Flecainide reduced the maximal interatrial conduction delay, but pilsicainide did not. Each drug increased the wavelength index in a tachycardia-dependent manner. Class Ic drugs may reduce atrial vulnerability by increasing the wavelength of the reentrant circuit during periods of rapid heart rate.