Blood flow studies in the brain, heart, and other organs suggest that there could be interaction between nitric oxide (NO) and adenosine. This possibility was investigated in the optic nerve head (ONH) during photic stimulation of the dark-adapted cat eye. Functional activation of ONH blood flow was measured by laser Doppler flowmetry, simultaneously with NO and PO(2) using double-barrel recessed electrochemical sensors. Photic stimulation (diffuse luminance flickering light at 30 Hz) increased ONH blood flow to 127.4 +/- 4.7% (mean +/- SEM) of baseline with a transient increase in NO by 79.8 +/- 12.8 nM, while PO(2) decreased from 24.5 +/- 2.7 to 22.7 +/- 2.4 Torr (control responses, 15 trials, 10 cats). Adenosine (3 mg/kg iv) increased baseline ONH blood flow to 113.8 +/- 8.4% of control within 5 min. Functional activation of ONH blood flow was enhanced during photic stimulation, reaching a maximum of 155.8 +/- 8.1% within 5 min, and remained enhanced for 30 to 45 min. NO responses during photic stimulation were not different from control responses. Treatment with a nonspecific NO synthase inhibitor (N(G)-nitro-L-arginine methyl ester, 40 mg/kg iv, 5 cats) did not alter the increase in resting ONH blood flow or the enhanced functional activation after adenosine. We conclude that there is no interaction between NO and adenosine during functional activation of cat ONH blood flow by photic stimulation.