Forty weanling male rats were administered 0.4% beta-aminopropionitrile fumarate (BAPN) for 3 weeks and were then fed an atherogenic diet supplemented with 0.2% BAPN for 4 weeks and with 0.l% BAPN for 9 weeks. Twenty control rats were fed the atherogenic diet without added BAPN. Grossly, intimal lipidosis of the aorta stained with Sudan IV appeared more extensive in test than in control rats. Histologically, intimal lipidosis of the aorta in test rats appeared as early as at 3 weeks on the atherogenic diet. Six test rats showed medial dissection of the thoracic aorta with intimal lipidosis. Dissected spaces of the aorta were filled with proliferating medial cells and, in the subendothelial zone, the smooth muscle cells were mixed with lipid-laden macrophages. Electron microscopically, the aortic intima of test rats showed lipid-laden macrophages and smooth muscle cells in widened subendothelial space. Between the test and control groups, none of the major serum lipid fractions showed significant difference.The results suggest that lathyritic changes of rat aortas induce not only active repair of damaged media but also enhance intimal lipid deposition.