Histological changes in the aorta were used as criteria to assess the effect of treadmill exercise on the development of atherosclerosis in rats consuming a moderately hyperlipemic diet. While grossly visible atherosclerotic plaques were absent, microscopic examination of sections of the abdominal aorta were distinctly different between exercised and sedentary rats. Aortas of sedentary rats fed a diet containing lard and cholesterol had a high degree of plaque development, fat accumulation, mineralization, erosion and necrosis. Aortas of exercised rats fed the same diet had considerably less atherogenic involvement. Both hyperlipemic groups had greater plaque formation than sedentary rats fed a normolipemic diet. In addition, there were marked differences in plasma lipids; exercise ameliorated the diet-induced changes in plasmatic atherogenic lipids. Our results indicate that exercise retards the development of atherosclerosis in rats fed a hyperlipemic diet.