The myocardium contains collagen matrix that is a major determinant of its architecture, structural integrity, and mechanical properties. This fibrillar matrix consists primarily of type I and type III collagens having epimysial, perimysial, and endomysial components. The present study shows the alteration of collagen phenotypes during the evolution of hypertensive hypertrophy. Therapy with captopril, an angiotensin-converting enzyme inhibitor that regresses cardiac hypertrophy, not only reduces the total amount of collagen but reverses the altered distribution of type I and type III collagen. In normotensive rats, captopril did not significantly reduce collagen content or alter the ratio of type I to type III collagen.