Recently the endothelium-derived relaxing factor was proved to be nitric oxide (NO) and it has been found to play an important role in the regulation of local blood flow. In this paper, we studied the effect of S-nitroso-N-acetyl-DL-penicillamine (SNAP), a NO donor, on the optic nerve head (ONH) circulation in rabbits, using a hydrogen gas clearance flowmeter etc. Intravitreal application of SNAP (20 nmol) increased the caliber of the retinal artery at the edge of the ONH 60 to 120 minutes later and capillary blood flow in the ONH 90 to 135 minutes later. A dose-response relation was found between 2 and 20 nmol. These changes were thought to be caused by NO produced from SNAP because they were inhibited by pretreatment with a NO trapping agent. Blood pressure and IOP were reduced 30 to 45 minutes later and 60 to 120 minutes later, respectively. It was considered that intravitreal injection of SNAP first reduced both the resistance of the ONH blood vessels and the perfusion pressure in the ONH and that the latter then rose, followed by an increase of the capillary blood flow in the ONH.