One to 3 injections of 40 units of adrenocorticotropin (ACTH)-gel significantly enhanced the elevated serum cholesterol and phospholipid levels produced by subsequent thiouracil and cholesterol feeding in the dog. This effect was associated with paralytic strokes in 4 of 10 dogs. Autopsy revealed markedly more widespread and more severe atherosclerosis in dogs pretreated with ACTH than in dogs who received only a high thiouracil and cholesterol diet.