Microcirculation in the optic nerve head (ONH) in the ocular fundus was measured noninvasively using the recently developed laser speckle circulation analyzer. Results were compared with capillary blood flow (CBF) measurements made by the hydrogen clearance method. The ONH blood flow of six normal rabbits was measured simultaneously by both methods. Inhalation of 10% carbon dioxide (CO2) increased the CBF by 22% (hydrogen clearance method). Intravenous injection of endothelin-1 (ET-1) (10(-10) mol/kg) decreased the CBF by 19%. Significant correlations were found between the NB (normalized blur) value by the laser speckle method and CBF (r = 0.38, P < 0.05, n = 30) and also between the relative change of NB and CBF (r = 0.92, P < 0.01, n = 30) in the ONH. This indicates that evaluation of the NB change in the ONH is as accurate with the laser speckle method as with the hydrogen clearance method, giving a basis for the clinical application of the laser speckle method of measuring ONH circulation.