The aim of the present study was to investigate whether metformin was capable of altering aortic lipid metabolism. Pretreatment of rabbits for 8 days with 120 mg/kg per os metformin reduced by 50--70% the incorporation of a 20 muCi tracer doseof [4(-14)C]cholesterol (given orally 24 h before) into various segments of aorta, plasma, liver, intestine and lung, as compared with control animals. However, as intestinal absorption of cholesterol was also diminished in the same proportion, it was then decided to inject the labelled cholesterol directly into the blood. Under these conditions, metformin induced the same reduction in [4(-14)C]cholesterol specific activity in the aorta, but not in other tissues. Three hours after intravenous injection of a 200 muCi tracer dose of [2(-14)C]-acetate, metformin strongly diminished the radioactivity of total lipids of the aorta, both in fed-rabbits and in rabbits on a 24 h fast, independently of the plasma radioactivity level. The inhibition of acetate incorporation into arterial lipids was observed in all lipid fractions (i.e. free and esterified cholesterol, free fatty acids, phospholipids and especially triglycerides) and the effect persisted unaltered over periods of increasing length after the injection of precursor (1/4, 1/2, 1, 3, 5, 8, 12 h). Metformin also significantly inhibited lipid biosynthesis in the liver and intestine. These properties, added to others previously described, can to a large extent explain the preventive effect of metformin on experimental atherosclerosis.