Previous studies have shown that experimental canine coronary or rabbit cardtid qrtery stenosis that is associated with endothelial injury results in a typical pattern of blood flow characterized by gradual decreases in arterial flow followed by restorations of flow to normal values. This pattern of flow, called cyclic flow reduction(CFR), is the consequence of recurrent platelet aggregation at the site of the stenosis and endothelial injury and subsequent dislodgement of the thrombus. This study was designed to test the efficacy of verapamil in ingibiting in vivo platelet aggregation in a rabbit model of cardotid artery stenosis and ecdothelial injury. Carotid blood flow was measured continuously with a electromagnetic flowmeter probe that is positioned proximal to the constrictor. During placement of constrictor and angioplasty balloon, CFR developed in 8 of 20 rabbits with a mean frequency of 10.0+/-2.2 cycles/h. CFRs were observed for 30min, and IV verapamil was administered till declining of blood pressure(up to 100ug/kg). After intravenous verapamil, the mean frequency of CFR insignificantly decreased to 8.7+/-2.1 cycles/h(p=ns). After 20mg/kg of aspirin were given intravenously, the CFR were abolished in 5 rabbits, the mean frequency of CFR decreased in 1 rabbit, and no significant change was observed in 2 rabbits. It is concluded that verapamil is relatively ineffective in inhibiting in vivo platelet aggregation at doses that don't change hemodynamics significantly.