Purpose: To evaluate whether prostaglandins are mediators of the hypercapnia-associated vasodilation of the optic nerve head vessels. Methods: We measured the PO<sub>2</sub> at intervascular areas of the optic disc in 9 anaesthetized miniature pigs using oxygen-sensitive microelectrodes placed at <50 µm from the optic disc. PO<sub>2</sub> was measured continuously under normoxia, hyperoxia (breathing of 100% O<sub>2</sub>), carbogen breathing (95% O<sub>2</sub>, 5% CO<sub>2</sub>), and hypercapnia (40% increase in inhaled CO<sub>2</sub>). Similar measurements under these conditions were also done after intravenous administration of the prostaglandin inhibitor indomethacin. Results: Before the injection of indomethacin, we observed a slight increase in the optic disc PO<sub>2</sub> during hypercapnia (ΔPO<sub>2</sub> = 2.0 ± 1.7 mm Hg; p < 0.001; n = 18) or hyperoxia (ΔPO<sub>2</sub> = 3.4 ± 1.6 mm Hg; p < 0.001; n = 23), but a much more important increase during carbogen breathing (ΔPO<sub>2</sub> = 12.0 ± 5.1 mm Hg; p < 0.001; n = 23). After the injection of indomethacin, the increase in the optic disc PO<sub>2</sub> was similar during hyperoxia (ΔPO<sub>2</sub> = 5.6 ± 2.2 mm Hg; p < 0.001; n = 9) or carbogen breathing (ΔPO<sub>2</sub> = 5.8 ± 3.2 mm Hg; p < 0.001; n = 9), while in hypercapnia the variation of the optic disc PO<sub>2</sub> was minimal (ΔPO<sub>2</sub> = 0.5 ± 1.9 mm Hg; p > 0.1; n = 6). Conclusions: Indomethacin inhibits the vasodilatory effect of increased systemic PaCO<sub>2</sub> on the optic nerve head vessels, leading to a similar moderate increase in the optic disc PO<sub>2</sub> during carbogen breathing as in hyperoxia. Indomethacin also inhibits the increase in the optic disc PO<sub>2</sub> seen during hypercapnia. Those results indicate that prostaglandins are mediators of the hypercapnia-associated vasodilation of the optic nerve head vessels.