Using conventional glass microelectrode technique, electrophysiological effects of flecainide acetate on guinea-pig left atrial muscle fibers were examined. Resting membrane potential was not affected by flecainide at any concentrations tested (10(-7) M-3 X 10(-5) M), although overshoot potential was significantly decreased at the concentration over 3 X 10(-6) M. Effective refractory period significantly increased at 10(-5) M. The reduction of Vmax was about 25% and 50% by 3 X 10(-6) M and 10(-5) M, respectively at 1 Hz. At 3 X 10(-6) M, Vmax decreased by about 10%, 15%, 22%, 33%, and 34% at 0.2, 0.5, 1, 2, and 3 Hz respectively. Flecainide, decreased Vmax of atrial muscle fibers in a dose and frequency dependent manner. It is suggested that flecainide might be effective in the treatment of atrial tachyarrhythmias.