Encainide (MJ 9067) was given intravenously at 1, 2, or 5 mg/kg and orally at 1, 2.5, 5, or 10 mg/kg to conscious dogs 18--23 h after two-stage ligation of the left anterior descending coronary artery. Heart rate and ventricular ectopic rate were lowered in a dose-related manner by 40--70 and 55--158 beats/min, respectively, following drug administration. Ventricular arrhythmia was frequently abolished with doses of 2 mg/kg and above. The onset of antiarrhythmic activity was 1--2 min after intravenous injection and 15--30 min after oral ingestion. The duration of effect was 6--7 h or longer following either route of drug administration. Emesis occurred in 1 of 10 and 1 of 6 dogs, respectively, at the 2 and 5 mg/kg intravenous dose levels. Brief ataxia was also observed in one dog given the latter dose.