The effectiveness and safety of quinidine in the conversion of chronic atrial fibrillation after administration of amiodarone was assessed in 15 patients. A total quinidine dosage of 1097 +/- 408 mg was administered up the point of conversion or for a total of 48 hours. Nine of 15 patients (60%) converted to sinus rhythm. No clinical variable such as the duration of atrial fibrillation, left atrial size, left ventricular fractional shortening, amiodarone duration, or maintenance dose of amiodarone was able to discriminate between converters and nonconverters when patients were treated with the combination of amiodarone and quinidine. The mean QT interval with amiodarone was 414 +/- 44 msec and slightly increased to 434 +/- 40 msec (p = 0.01) when quinidine was added. The amiodarone-quinidine combination was well tolerated, and no side effects or proarrhythmias were recorded.