Chanutin (with Ferris1 and with Barksdale2) has recently shown that in rats deprived of 80 per cent of their renal tissue progressive renal lesions, arterial hypertension and cardiac hypertrophy develop. The width of the myocardial fibers of the right ventricle does not change, while the diameter of the fibers of the left ventricle bears "a substantial relationship" to the ratio of the weight of the heart to the surface area. The comparison of a unidimensional or bidimensional method of measurement with a volumetric method has in the past led to serious misinterpretation of data. In this instance there has been no error in conclusions, but the comparison of width of the fibers (a linear measurement) with the weight of the heart (proportional to the total volume of all the fibers) has concealed an important fact, namely, that when the fibers of the heart of the rat hypertrophy