Anterograde block in the accessory pathway with a short effective refractory period was observed after intravenous (IV) injection of 1 mg/kg of ajmaline in two patients with the Wolff-Parkinson-White syndrome. The possibility of a false-negative response to this test is discussed. Ajmaline is proposed as an emergency drug in the Wolff-Parkinson-White syndrome in the setting of atrial fibrillation with a very high ventricular rate.