Repeatedly-bred rats of both sexes develop subintimal accumulations of acid mucopolysaccharides and fibroblastic proliferation followed by distortion and destruction of elastic lamellae. Fibrosis, endothelial hyperplasia, lipid deposition, calcification and atheromata are also observed. Changes in the elastic tissue occur independently, and later than the alterations of the ground substance and the fibroblasts. Calcification is found in more advanced lesions, showing an affinity for areas rich in mucopolysaccharides. Exogenous ACTH and unilateral nephrectomy augment early atherosclerotic changes associated with multiple breedings.