In all conduction disorders of the left ventricle the Q-T interval is prolonged in relation to the prolongation of the QRS duration, but the S-T interval (measured from the end of the S wave) is within normal limits. Right ventricular conduction defects are associated with prolongation of the Q-T interval, but the S-T interval in incomplete and complete right bundle branch block is subnormal. In the absence of a broad QRS, the Q-T and S-T intervals are not prolonged in left anterior hemiblock. However, an associated non-specific left intraventricular conduction defect or right bundle branch block does prolong the QRS and thus the Q-T interval. The S-T interval is normal or subnormal in the presence of right bundle branch block and left anterior hemiblock confirming that left anterior hemiblock does not affect the Q-T interval and repolarisation. In the presence of any conduction defect which prolongs the QRS duration, extrasystoles have longer coupling intervals. The coupling intervals are longer in the presence of left bundle branch block than right bundle branch block. It is postulated that this is due to the differences in the time of depolarisation and thus repolarisation of the left ventricle.