[en] EO-199, a demethylated analog of the novel class I antiarrhythmic drug EO-122 was found to antagonize the antiarrhythmic activity of EO-122 and that of procainamide (Class I_A). EO-199 did not block significantly the activity of a class I_B antiarrhythmic agent, lidocaine. EO-199 also displaced the specific binding of [³H]EO-122 to rate heart membranes similarly to procainamide whereas lidocaine did not. The correlation between binding experiments and pharmacological effects points to a possible subclassification of these drugs; the two chemical analogs EO-199 and EO-122, as well as procainamide (I_A) but not lidocaine (I_B), compete at the same site or the same state of the sodium channel. The availability of a specific antagonist might be useful for studying the mechanism of action of antiarrhythmic drugs as well as an antidote in cases of antiarrhythmics overdose intoxication