A study was made on the effect of simvastatin (the generic name of MK-733), a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, on coronary atherosclerosis in cholesterol-fed rabbits with focus on the serum lipids and morphology. Twenty-seven Japanese white rabbits were divided according to dosage of simvastatin into four groups as follows, group P (placebo, 5 rabbits), group MK 1 (simvastatin 1mg/kg, 5 rabbits), group MK 3 (simvastatin 3mg/kg, 6 rabbits) and group MK 5 (simvastatin 5mg/kg, 5 rabbits). They were placed on a 0.5% cholesterol atherogenic diet for 16 weeks and measurements were made of the concentration of serum lipids weekly. After sacrifice, the degree of surface involvement (SI) of aorta stained with Sudan III and the degree of coronary stenosis (CS) of the left circumflex artery were measured using an image-processing system. Serum total cholesterol (TC) level and beta-lipoprotein level decreased dose-dependently in MK groups compared with group P. High density lipoprotein cholesterol level increased in groups MK 3 and MK 5 slightly. Triglyceride level decreased in groups MK 3 and MK 5. The progressions of SI and CS were suppressed in MK groups dose-dependently. Integrated TC, that is, sum of the serum TC values obtained at each week multiplied by 7 corresponded more closely to CS than SI. Intimal thickening constructed from large foam cells originated from macrophages and proliferating smooth muscle cells included lipid droplets in MK groups was almost similar in group P. But it was likely that lipid droplets in each smooth muscle cell in MK groups were less than in group P. In conclusion, the development of coronary atherosclerosis in cholesterol-fed rabbits was suppressed dose-dependently by simvastatin and it was suggested that this preventive effect was due to reducing the integrated TC and local action to vessel walls by simvastatin. (Fukuoka Acta Med.)