WITH the development of computer techniques applied to electrocardiographic analyses, emphasis has been shifted to quantitative rather than qualitative interpretation of the electrocardiogram. Conduction defects represent one of the areas of electrocardiography where quantitation is most desirable. It has been demonstrated that neither the duration of the QRS interval nor the subjective estimation of the slurrings observed in the QRS complex permits an accurate quantitation of progressive degrees of right bundle-branch block. 1-4 The study of the spatial velocity of the ECG might represent an optimal method for quantitative studies of ventricular conduction defects. 5-6 The object of this investigation was to study the effects of variable degrees of right bundle-branch block on the velocity of directional changes in the orthogonal ECG. Material and Methods Right bundle-branch block occurred in two patients during the course of right ventricular catheterization. The first patient (case 1) was a 6-year-old child with a