Clinical and electrophysiologic data from 51 consecutive patients with sustained monomorphic ventricular tachycardia inducible during programmed ventricular stimulation were evaluated to determine what variables predict the response to intravenous class IA antiarrhythmic agents. All patients received acute drug testing in the electrophysiologic laboratory with either intravenous procainamide or intravenous quinidine. Ventricular tachycardia suppression was achieved in 9 out of 51 patients (18%). The age, gender, left ventricular ejection fraction, baseline right ventricular effective refractory period, baseline HV interval, and baseline ventricular tachycardia cycle length were not predictive of ventricular tachycardia suppression with intravenous procainamide or quinidine during programmed ventricular stimulation. The degree of prolongation of the right ventricular effective refractory period after drug administration did not predict success or failure to suppress inducible ventricular tachycardia. The degree of prolongation of the HV interval was also not predictive. In addition, the degree of prolongation of the right ventricular effective refractory period or the HV interval did not predict the change in the ventricular tachycardia cycle length after drug administration in patients who remained inducible. These data indicate that the response to class IA antiarrhythmic agents in patients with inducible sustained monomorphic ventricular tachycardia cannot be predicted on the basis of various clinical and electrophysiologic parameters.