Background In 1952, Prinzmetal induced preexcitation in the normal dog heart using subthreshold stimulation (SS) delivered to the right ventricle.Methods and Results In 12 dogs we recorded ECG leads II, aVR, His (Hb) and proximal right bundle potentials with electrode catheters at the aortic root and a special electrode that was inserted through the right ventricular (RV) free wall.In 12 others, SS was delivered to the Hb area by a catheter placed under the septal leaflet of the tricuspid valve. During SS, the HV interval shortened from 35+-4 milliseconds (mean+-SD) to 19+-7 milliseconds (P=.0001), but AH intervals were unchanged. The ECG showed delta waves with aberrant QRS complexes. Endocardial electrograms showed that the origin of activation in the preexcitation beats was localized to the muscle adjacent to the Hb or proximal right bundle. When vagal stimulation induced sudden AV block, no ventricular excitation was seen, confirming the subthreshold nature of the applied stimulation. By adjusting the levels of SS, latent forms of preexcitation could be induced, eg, early local septal muscle activation but no change in the ECG leads. Premature ventricular stimuli delivered to the RV apex or outflow tract could cause manifest preexcitation in the ECG leads or inhibit expression of latent preexcitation in endocardial recordings.Conclusions SS delivered to the RV apex or Hb area causes ventricular preexcitation, as shown previously by Prinzmetal et al.SS delivered at the insertion sites of an accessory pathway may facilitate localization of such abnormal connections, particularly when preexcitation is concealed. (Circulation. 1994;89:2380-2389.)