The growth of the muscular and collagenous parts of the rat heart in various forms of cardiomegaly.

1. Cardiomegaly has been produced in rats by sideropenic anaemia, by isoprenaline or thyroxine or by the application of both drugs, by artificial increase in resistance to blood flow and by long-term adaptation to hypoxia and physical stress. The ratio of the growth of muscle to the growth of collagen in the heart has been studied.2. All possible variations in the ratio occurred depending on the type of stimulus used for inducing cardiomegaly and on the dynamics of the development of cardiomegaly. In cardiomegaly induced by sideropenia and by thyroxine the growth of muscle was not accompanied by the growth of collagen. Exposure to hypoxia or isoprenaline administration increased only the growth of collagen in the hypertrophic heart. In all other forms of cardiomegaly muscle and collagen formation were stimulated to the same extent.3. It is concluded that when certain organs hypertrophy during adult life several factors may determine the relative rapidity of growth of the muscular or parenchymal and the collagenous stromal components of the tissue.