To estimate the effect of dilazep in preventing restenosis after a transluminal angioplasty, we have attempted an animal experiment in which the efficacy of drug was tested on an angioplastic injury superimposed on to atherosclerotic lesions. Using a modification of Block's method, atherosclerotic lesions first were made in the rabbit aorta by an initial ablation of the endothelium, followed by successive cholesterol feedings. Then, for our second step, an angioplastic injury was inflicted on the atherosclerotic lesions. The thirty two rabbits used were divided into 3 groups: those given dilazep (100 micrograms/kg i.v.), those given dipyridamole (100 micrograms/kg i.v.) and a control group that was given the same volume of 0.9% saline. The angioplastic area in which thrombi developed was semiquantitatively measured and compared among all three groups. The mean value of the thrombus area in the dilazep group was 60% smaller and the distribution pattern of the thrombus by size was found to be composed of smaller thrombi than those of the control group. Dipyridamole showed the same trend as did dilazep, but less effectively. Our animal model and semiquantitative evaluation method that we employed were found useful from the view point of an easy applicable and inexpensive methodology. Our results point towards the possible clinical use of dilazep in the future, so as to prevent thrombus formation, which seems to cause the restenosis phenomenon that often occurs in patients who have undergone a coronary angioplasty.