Mitochondrial abnormalities are the most common ultrastructural disturbance reported in the heart during diabetes. Ultrastructural derangement in the myocardium does not always accompany diabetes mellitus. Several factors have been identified in the diabetic state which affect the development of ultrastructural irregularities in the heart. One of the more closely examined factors which influence the progression of ultrastructural damage in the heart is the length of time that the animal is exposed to diabetes. The severity of the diabetes may represent a plausible explanation for the presence or absence of ultrastructural changes in the heart during diabetes. A relationship of the ultrastructural abnormalities in the heart during diabetes to cardiac functional depression has been challenged by one study; however, the existence of such damage does provide clear, undeniable evidence that the heart is undergoing a process of pathological regression. Sarcomere disorganization, myofibrillar loss, and mitochondrial swelling and clearing are among of the more common ultrastructural lesions reported.