The effect of aspirin on experimentally induced atherosclerosis was studied in rabbits. Rabbits were placed on an atherogenic diet containing either no aspirin or 0.2% aspirin supplement. Control rabbits were fed regular rabbit food or rabbit food supplemented with 0.2% aspirin. Ingestion of aspirin from the diets containing aspirin was equivalent to a daily dose of 100 mg/kg. As expected, 2 mo after the rabbits were placed on the atherogenic diet, extensive atheromatous lesions were observed on gross examination sporadically distributed along the walls of the aorta. The coronary arteries also exhibited atheromatous lesions on microscopic examination. Addition of aspirin to the atherogenic diet intensified the atherosclerosis as measured by proliferation of the intima of the aorta and coronary arteries and increased occurrence and distribution of atheromatous plaques. It is concluded that, under the conditions of this experiment, the addition of aspirin to an atherogenic diet greatly intensified atherogenesis.