Loose silver rings were placed on the descending aorta of growing rats. During 5 months'' resistance cardiomegaly (36%) occurred. This cardiomegaly was characterized by a significant decrease in the number of both capillaries and muscle fibers/mm2 and a prolongation of the diffusion distance (D/2). The lower F/C R indicates a slight increase in the number of capillaries. The effect of the age of the animal on the character of cardiomegaly is discussed: cardiomegaly arising very early in life is much greater than that described here and yet the diffusion distance is not prolonged and fibers do not undergo hypertrophy.