The platelet glycoprotein (GP) IIb/IIIa receptor antagonist appears to reduce the need for revascularization after coronary angioplasty. However, since the effect of GP IIb/IIIa receptor antagonist on the in-stent neointimal thickening has not been clarified, we examined it in the canine model. The beagle dogs were assigned to the control (n=7) or the GP IIb/IIIa receptor antagonist FK633 group (n=7). FK633 was administered by subcutaneous osmotic pumps (0.2 mg. kg-1. h-1) and an intravenous bolus injection (1 mg/kg) before stenting. A coil stent was implanted in the left circumflex coronary arteries. The platelet aggregation capability was significantly (<5%) and consistently reduced by FK633 except for the mild elevation (10% to 30%) on the next day of stenting. Hearts were excised 3 months after stent implantation. The area of intima and media and the area stenosis were obtained from the sections of the stented arteries. The area of intima and media and the area stenosis (1.3+/-0.2 mm2, 41.8+/-7.5% and 1.3+/-0.2 mm2, 33.9+/-6.7% in the FK633 and the control group, respectively) were not different between the groups. We conclude that, although GP IIb/IIIa antagonist FK633 prevented the platelet aggregation significantly and consistently, it could not prevent the neointimal thickening after stent implantation in canine coronary artery.