Groups of rabbits were fed isonitrogenous diets containing 30% soya, milk or whitefish meal protein and at least 130 g/kg of added fat for 1 year. Mean serum cholesterol values in fish meal animals (13.4 +/- 2.4 mmol/l) were substantially greater than for soya (3.0 +/- 0.2 mmol/l) or milk (4.6 +/- 0.7 mmol/l). Fish meal rabbits developed extensive aortic atherosclerosis (c. 70% surface involvement) which histologically showed both fibrous and foam cell intimal thickening and destruction and calcification of medial elastic tissue. In a second experiment fish meal was given with either saturated (coconut oil) or unsaturated (maize oil) fat. A similar degree of hypercholesterolaemia developed in each group and was rapidly reversed when a soya protein-low fat diet was substituted. This model may therefore be of value in studies of the progression and regression of experimental atherosclerosis.