The efficacy of intravenous procainamide, infused at 30 mg/min to a maximum dose of 20 mg/kg, in converting atrial fibrillation was evaluated under electrocardiographic and blood pressure control in 21 patients. Nine patients had atherosclerotic heart disease, seven had valvular lesions, five had hypertension, and six had no apparent heart disease. Nine patients (converters), who reverted to sinus rhythm at a mean dose of procainamide of 13.3 +/- 3.6 mg/kg (mean plasma concentration, 7.4 +/- 3.9 micrograms/ml) had normal echocardiographic left atrial diameters. All but one of the remainder, the 12 non-converters, who received a mean drug dose of 13.1 +/- 3.5 mg/kg (mean plasma concentration 13.9 +/- 7.6 micrograms/ml), had atrial diameters exceeding 4.0 cm. QRS and QTc intervals were not altered significantly in converters, but were prolonged significantly in non-converters. No serious side effects from the infusion were encountered. Intravenous procainamide infusion appears to be a safe and rapidly effective method of converting recent-onset atrial fibrillation to sinus rhythm in patients with normal left atrial dimensions.