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Procainamide is a class IA antiarrhythmic drug indicated for the treatment of life-threatening or symptomatic ventricular arrhythmias. The current sustained-release formulation requires 6-hour dosing (qid). To improve patient compliance, a new sustained-release formulation for twice-daily (bid) administration has been developed (Procanbid, Parke-Davis). This study assesses the pharmacologic equivalence of the bid and qid formulations in the suppression of symptomatic ventricular premature depolarizations (VPDs). Fourteen centers enrolled a total of 99 patients with frequent symptomatic VPDs (average ≥20 VPDs/hr) who previously responded to and tolerated the procainamide qid formulation. During the first week of the double-blind phase, patients were randomized to either placebo or procainamide dosages of 1000, 2000, or 4000 mg/d (bid or qid formulations). In the second week, the patients were crossed over to the alternate formulation. Seventy-seven patients qualified for the primary activity analysis. The bid and qid formulations showed comparable effectiveness in the suppression of mean VPDs with a linear dose-response relationship. The VPD suppression was not attenuated towards the end of the dosing interval for either formulation. Sixty-eight of these patients entered an optional 1-year extension to receive the bid formulation. Thirty-seven (54%) patients had adverse effects. Of those, 15 (22%) had side effects considered treatment related. Most of the adverse events occurred during the first 6 weeks of treatment. Only a few patients (8%) withdrew as a consequence of treatment with the bid formulation. The overall safety profile of the bid formulation was similar to other formulations, and the procainamide bid formulation has a low proarrhythmic rate (`3%). In conclusion, the effectiveness of the twice-daily formulation of procainamide in the suppression of VPDs is comparable to the currently available qid formulation.