The extreme prolongation of ventricular action potential duration that occurs in some of the long QT syndromes may result in two forms of alternating activity of the heart: a "pseudo" 2:1 atrioventricular (AV) block and a T wave alternation, both of which are rate dependent. The pseudo 2:1 AV block relates to the extreme prolongation of ventricular refractoriness. The T wave alternation reflects the fact that the rate dependence of action potential duration differs in degree or magnitude in the subendocardial and subepicardial layers of the ventricular wall. Examples of two cases previously reported in the Journal by Weintraub et al. are used to illustrate and discuss these manifestations.