Disturbances in retinal perfusion due to impaired regulation of vascular tone are believed to be involved in the pathogenesis of several vision-threatening retinal diseases. Two recent studies have shown that the glutamate receptor agonist, N-methyl-D-aspartic acid (NMDA), and adenosine induce relaxation of isolated porcine retinal arterioles in vitro. However, it remains to be elucidated whether the relaxing action of the two substances are coupled.Porcine retinal arterioles with preserved perivascular retinal tissue were mounted in a myograph for isometric tone measurements. Changes in tone were induced by increasing concentrations of NMDA in the presence of blockers of adenosine receptors and ATP hydrolysis and by increasing concentrations of adenosine in the presence of the NMDA receptor blocker DL-APV (DL-amino-5-phosphonovaleric acid). The experiments were repeated after the perivascular tissue had been removed.NMDA produced a relaxing effect on retinal vessels with preserved perivascular retinal tissue (P < 0.001) which disappeared after removal of the tissue. Blocking of the NMDA and adenosine receptors and hydrolysis of adenosine triphosphate (ATP) significantly reduced the vasorelaxing effect of NMDA in the presence of perivascular retinal tissue (P < 0.05 for all three comparisons). Adenosine produced a concentration-dependent relaxation that was not significantly affected by blocking the NMDA receptor with DL-APV (P = 0.088).The findings suggest that the vasorelaxing effect of NMDA on porcine retinal arterioles in vitro is mediated by hydrolysis of ATP to adenosine in the perivascular retinal tissue.