Rapidly replicating tissues such as malignant neoplasms accumulate porphyrins and often display a typical fluorescence. Previous authors observed a selective fluorescence due to injected porphyrins also in atheromatous plaques, while no fluorescence was observed in the plaque-free arterial wall. We have performed quantitative studies on the distribution of haematoporphyrin in the broad-breasted white turkey, in which hypertension and atherosclerosis occur spontaneously. In this experimental model the aortic atherosclerotic lesions are confined to the abdominal tract, and the thoracic tract exhibits a hypertrophic media. After haematoporphyrin injection, larger porphyrin concentrations were observed in the inner than in the outer portion of the abdominal aorta. An opposite behaviour was observed in the thoracic tract. Notably, haematoporphyrin was progressively cleared from the thoracic media, while its concentration remained relatively stable in the inner portion of the abdominal aorta up to four days after the injection. This indicates a selective and prolonged retention of haematoporphyrin within the atherosclerotic lesions.