Abstract Thirty vectorcardiograms of left bundle branch block were studied. The time, direction, sense and position of the Q, R and ST-T loops were studied. Twenty-five cases had QRS loops with greater duration than 120 milliseconds and five ranged from 100 to 120 milliseconds. The Q loop was present in six cases. Nineteen of the vectorcardiograms had a characteristic initial component directed anteriorly and to the left. In five vectorcardiograms the initial part was the centrifugal limb of the R loop. The R loop rotated clockwise in the horizontal and sagittal planes and counterclockwise in the frontal plane. It was directed to the left, posteriorly and superiorly. Dividing it into initial component, centrifugal limb, plateau and centripetal limb, the two last portions were the most delayed. The initial component was very characteristic of this condition. The plateau correlated very well with the total delay. The longer the QRS loop, the longer the plateau and the more frequent its occurrence. The J point and the T loop were always opposite to the R loop. In general, important torsion of the T loop was found, indicating secondary changes. In most of the cases the T loop had a slow centrifugal limb and a rapid centripetal limb. A correlation with ventricular activation and an explanation of the vectorcardiographic behavior were given.