Vervet monkeys (Cercopithecus aethiops pygerethrus) were fed cholesterol-free, semipurified diets containing 40% sucrose, 25% casein, 15% cellulose and 14% peanut oil (PNO), randomized peanut oil (RPNO) or corn oil (CO). After 4 months, serum cholesterol and triglyceride levels, serum lecithin-cholesterol acyl transferase (LCAT) activity and plasma lipoprotein lipase (LPL) activity were similar in all groups. Livers of monkeys fed CO converted 156% more acetate and 24% more mevalonate to cholesterol than those of monkeys fed RPNO. Cholesterogenesis in RPNO-fed monkeys was enhanced compared to PNO (68% from acetate; 62% from mevalonate). Incidence of atherosclerosis was 33% in monkeys fed RPNO, 80% in those fed CO and 90% in those fed PNO. Extent of sudanophilia was lowest in aortas of monkeys fed RPNO. Incidence of arteriosclerosis was 40% in monkeys fed CO, 56% in those fed RPNO and 70% in those fed PNO. Extent of aortic surface showing arteriosclerosis was highest in monkeys fed RPNO.