A total of 1342 electrocardiograms demonstrating intraventricular conduction defects were reviewed. Criteria for distinguishing different types of conduction defects and combinations of such defects are presented. The time of intrinsicoid deflection was of great value and may be used to distinguish lesions of the main fascicles and peripheral Purkinje conduction defects. However multiple peripheral defects affecting a large area of myocardium can produce a pattern identical to left bundle branch block. It is suggested that intrinsicoid deflection time should be measured from the beginning of the QRS to the onset of the longest downslope at or after the peak of the R wave. It should be carefully related to the voltage of the dominant wave of the QRS complex in each lead. If intrinsicoid deflection is delayed in a limb lead that does not show the maximum voltage of the QRS, a peripheral left ventricular conduction defect is present; but if the lead does show the maximum voltage of the QRS, the defect may be central or peripheral. Another useful electrocardiographic feature was the direction of both the initial and terminal QRS forces in the 12-lead electrocardiogram. Such analysis often resolved the diagnosis of hemiblocks when the axis was indeterminate.