Electron-microscopic examination of aortas of rat treated orally and chronically with β-aminopropionitrile (BAPN) has shown progressive lesions in the elastic framework. This is associated with modifications of cells in the media which dedifferentiate into histiocytes and fibrocytes and also multiply. These changes begin in the 5th week of treatment, and are responsible for the hyperplasia of the intima as well as its thickening and collagenization. They support observations made in man during aortic aging, namely with regard to the intima and the intima-media junction. A high lipid diet given at various times during intoxication causes lipid deposition in the intima and a substantial penetration into the media when the lesions are developing. This also corroborates some hypotheses on the development of human atheroma. The addition of pyridinol carbamate (PDC) to BAPN prevents the formation of elastic tissue lesions. When given after the cessation of lathyrism treatment, PDC arrests and causes regression of lesions in the aortic media. It has no effects on lipid deposits, but the deposits are confined to the intima only