12 patients with normal QRS either in basal condition or during atrial pacing, undergone on single premature atrial stimulation, showed several types of intraventricular aberrancies. These have been revalued with atrial premature stimulation introduced over a progressively increasing frequency of fixed atrial stimulation (extrastimulus method), sometimes after atropine. In 9 cases it was possible to eliminate completely the above mentioned aberrancies, having thus a clear reduction of the refractory periods regarding their responsible structures. In 3 cases this was not possible, owing to a less significant reduction of the refractory periods. Such fenomenon is considered as the only parameter able to identify probable initial organic facts of the His-Purkinje system.