At age of 40 weeks, 9 WHHL rabbits were necropsied as a baseline control group for evaluating aortic and coronary atherosclerosis. An additional 18 WHHL rabbits of the same age underwent either a partial ileal bypass (PIB) or a sham operation. Serum cholesterol concentrations were significantly lower in PIB rabbits than in sham-operated rabbits, mean postsurgical values being 7.9 +/- 2.3 mmol/l and 13.2 +/- 2.4 mmol/l, respectively. This effect was mainly due to a lower LDL-cholesterol concentration. Serum triglycerides remained unaffected. Thirty weeks after surgery the PIB and sham rabbits were necropsied and atherosclerosis was evaluated. Progression of atherosclerotic plaque size in the sham group was most prominent in the aortic arch. PIB largely prevented this progression. The frequency of plaque complications in the aortic arch also increased in the sham groups. Here, no significant improvement was found in the PIB group. Extensive examination of the heart showed some affected coronary arteries in every 40-week-old WHHL rabbit. Progression of coronary atherosclerosis during the postsurgical period was small in both sham and PIB groups. However, while PIB did not affect the plaque size, it significantly reduced the number of coronary arteries affected.