A total of fifty-five 4 to 36 month old chickens were studied: 6 pullets, 28 layers and 27 non-layers. The plasma levels of triglyceride and cholesterol of the non-layers ranged from two to three fold and two to seven of the layers respectively after pubescence. The serum triglyceride levels decreased in the aged hens. More severe intimal thickening was seen in the coronary arteries of non-layers than those of layers. The non-layers had more severe lipid-rich lesions of the whole aorta and coronary artery. They showed a greater variety in the cellular composition of foam cells and hematogenous cells than the layers. Generally, lipid deposits were more frequent in the connective tissue cells than in the smooth muscle cells of the ascending and proximal thoracic aorta of non-layers. Continuous lipid permeation, provoked by extreme endogenous hyperlipidemia, appeared to be an important factor in the development of coronary and aortic lesions in hereditary non-laying hens.