Mechanisms coupling blood flow with changes in neuronal activity (functional hyperemia) in the brain, eye, and other organs are not completely understood. Nitric oxide (NO) has been suspected to play an important role, especially in the brain (Iadecola, 1993). We have shown that optic nerve head (ONH) blood flow increases during stimulation of the cat eye by flickering light, depending on the frequency and luminance of the stimulus, using a microscope-mounted infrared laser Doppler system to measure relative changes in ONH blood flow simultaneously with changes in extracellular K⁺ measured with ion-selective microelectrodes in the vitreous humor immediately above the ONH (Buerk et al., 1995). We have also measured changes in ONH blood flow simultaneously with NO near the ONH using recessed electrochemical microsensors (Buerk et al., 1996). We found that NO increased during neuronal activation by flickering light, and that the functional hyperemia in ONH blood flow and NO increases were significantly attenuated after inhibiting NO synthase. In this report, we describe simultaneous ONH blood flow, NO and PO₂ measurements during increased neuronal activity with flickering light. Preliminary results are consistent with coupling of functional hyperemia with increased O₂ consumption, but multiple interactions with NO need to be taken into account.