For comparison with our previous study on early calcification in human atherosclerosis, the aortas of rabbits and chickens with experimentally induced atherosclerosis were studied by gross examination, light microscopy and electron microscopy, including various cytochemical techniques. Nine male New Zealand white rabbits and nine male white leghorn chickens were fed an atherogenic diet of chow with 8% peanut oil and 2% cholesterol for one, two or three months. Six rabbits and six chickens, fed normal chow for one, two or three months, served as controls. The normal diet chickens were found to have lipid-negative spontaneous fibrous plaques in the abdominal aorta, which following atherogenic diet developed lipid deposition and increasing calcium deposition. The normal diet rabbits had no aortic lesions, but following an atherogenic diet developed highly lipid-positive foam cell intimal lesions which subsequently developed increasing amounts of smooth muscle cells and calcium. Ultrastructurally, the aortic plaques in normal diet chickens were composed of smooth muscle cells, collagen, elastic fibers, ground substance and a few small extracellular matrix vesicles bounded by a trilaminar membrane. In the atherogenic diet chickens, these vesicles increased in number as did their staining for calcium by the pyroantimonate technique. The membranes of vesicles were cytochemically positive for alkaline phosphatase and adenosine triphosphatase. Similar matrix vesicles were present in the interstitium of the media. In both intima and media, the vesicles appeared to be largely derived from degenerating smooth muscle cells. The aortas of atherogenic diet rabbits were similar to the chickens except for many more lipid-laden foam cells.(ABSTRACT TRUNCATED AT 250 WORDS)