Muscle fiber orientation in the left ventricular myocardial layer was histometrically estimated in normal, concentric and eccentric hypertrophied hearts. The angle of inclination of muscle fibers from coronal section was largest in the innermost and outermost zones and was progressively diminished toward the middle layer in all the hearts. In the inner layer, the inclination was depressed in eccentric hypertrophy, while there was no difference in fiber orientation in the outer layer. Concentric hypertrophy proved to be an intermediate stage between the normal heart and eccentric hypertrophy, and no opposite behavior was found in the arrangement of heart muscle fibers. The results supported the following view on the mechanism in the activity of hypertrophied hearts. Cardiac hypertrophy is a common accommodation process to increased load, regardless of the difference in the macroscopical shape. In hypertrophy, the ventricular cavity is inevitably enlarged. Consequently, the tension exerted on the ventricular wall on account of intraventricular pressure increases, and the contractile force required from a unit thickness of the ventricular wall is enhanced. The muscle fibers meet this increased demand by starting contraction from an over-stretched state and by ceasing its activity after smaller shortening than in the normal heart.