&NA;Isoproterenol treatment leads to endomyocardial fibrosis with muscle fibers encircled by fibrillar collagen. This study was undertaken in the rat to determine if muscle encased in collagen would subsequently become either necrotic or atrophic. For this purpose, we monitored the fibrillar nature of myocardial collagen, its alignment with muscle, and the morphology of the endomyocardium, together with the response in diastolic and systolic myocardial stiffness, immediately on completion (10 days) and 30 days after a course of subcutaneous isoproterenol (500 &mgr;g/kg/day). We found 1) left ventricular hypertrophy at 10 and 30 days with an increase in collagen volume fraction (p<0.01) that consisted of a meshwork of thick and thin collagen fibers that encircled endomyocardial muscle, 2) a variable reduction in endocardial muscle fiber diameter at 30 days with the greatest thinning seen in muscle encircled by fibrous tissue, and 3) an elevation (p<0.01) in the slope of the diastolic stress‐strain relation at 10 and 30 days. The developed systolic stress‐strain relation, which was elevated at 10 days (p<0.01), declined (p<0.05) with the reduction in endomyocardial muscle mass. Thus, endomyocardial muscle, encircled by fibrillar collagen, will atrophy over time, and this leads to a reduction in active stiffness. These findings may, in part, explain why progressive ventricular dysfunction accompanies chronic myocardial disease with endomyocardial fibrosis. (Circulation Research1989;65:258‐264)