Beagle dogs, 24 +/- 6 months old, fed a thiouracil-free semi-synthetic diet containing hydrogenated coconut oil and cholesterol (SS diet) for 12 months, developed marked hyperlipidemia and severe atherosclerosis. SS diet produced a marked elevation of serum cholesterol, triglyceride, phospholipid, and beta-lipoprotein and severe atherosclerosis in large and small arteries. Intimal fatty lesions were always present in the abdominal aorta and many of its branches. Large and small coronary arteries showed similar lesions. The degree of atherosclerosis was directly related to circulating lipid levels. Dextrothyroxine, at dose levels of 0.1 (equivalent to normal human dose) and 0.5 mg/kg body weight, produced a significant dose related lowering of serum lipids and was associated with a markedly decreased severity of aortic and coronary artery lesions. Untreated control dogs that were maintained on purina dog meal developed neither hyperlipidemia nor atherosclerosis.