Nineteen dogs were given131I and later administered appropriate amounts of methimazole or propylthiouracil daily. Over a period of 4-7 years several developed severe clinical features of myxedema. The myocardium and skeletal muscle of these severely myxedematous dogs displayed an array of morphologic changes on an ultrastructural level. Particularly interesting was the two- to threefold increase in the thickness of the myxedematous capillary basement membranes over those of normals. Increased tortuosity, lamination, and perivascular fibrosis were also observed as part of the myxedematous changes. These studies also demonstrated severe microcellular modifications, such as mitochondrial disruption, loss of cristae, lipid inclusions, and the presence of myelin figures, all a part of the diffuse systemic cardiovascular connective tissue pathology in myxedema.