A patient with heart failure was administered d-sotalol 3.0 mg/kg infused over 2 minutes. The patient had normal electrolytes and baseline QT. Six minutes after drug administration the QT prolonged to 600 msec, and the patient developed torsades de pointes and required electrical cardioversion. This emphasizes the need to consider both rate of administration and the dosage when evaluating the safety and efficacy of a new class III antiarrhythmic drug.