Myofibrillar disarray has been observed in various normal, experimental, and pathologic conditions of the heart. In order to elucidate the common denominator involved in the development of disarray, we undertook the present study to test the hypothesis that myofibrillar disarray can be produced in the normal heart as a result of altering myocardial stress vectors. In the heart, the vectors associated with the patterns of contraction are determined by the sequence of depolarization. In order to alter this sequence, the conduction system of 12 normal dogs was interrupted by the production of complete atrioventricular block. After 3 months of electronic pacing of the right ventricular apex at 70 bpm, myofibrillar disarray was observed in 9 out of 12 of these hearts. In matched tissue samples from 12 control dogs, myofibrillar disarray was not observed. We conclude that myofibrillar disarray may be produced by abnormal stress vectors resulting from alterations in the sequence of depolarization, and therefore the pattern of contraction. We believe this to represent a unifying concept to explain the development of myofibrillar disarray observed in a variety of conditions.