Reactive oxygen species (ROS) have been proposed to mediate vasodilation in the microcirculation. We investigated the role of ROS in arachidonic acid (AA)-induced coronary microvascular dilation. Porcine epicardial coronary arterioles (110 ± 4 μm diameter) were mounted onto pipettes in oxygenated Krebs buffer. Vessels were incubated with vehicle or 1 mM Tiron (a nonselective ROS scavenger), 250 U/ml polyethylene-glycolated (PEG)-superoxide dismutase (SOD; an \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}_{2}^{-}\) \end{document} scavenger), 250 U/ml PEG-catalase (a H₂O₂ scavenger), or the cyclooxygenase (COX) inhibitors indomethacin (10 μM) or diclofenac (10 μM) for 30 min. After endothelin constriction (30–60% of resting diameter), cumulative concentrations of AA (10^–10–10^–5 M) were added and internal diameters measured by video microscopy. AA (10^–7 M) produced 37 ± 6% dilation, which was eliminated by the administration of indomethacin (4 ± 7%, P < 0.05) or diclofenac (–8 ± 8%, P < 0.05), as well as by Tiron (–4 ± 5%, P < 0.05), PEG-SOD (–10 ± 6%, P < 0.05), or PEG-catalase (1 ± 4%, P < 0.05). Incubation of small coronary arteries with [³H]AA resulted in the formation of prostaglandins, which was blocked by indomethacin. In separate studies in microvessels, AA induced concentration-dependent increases in fluorescence of the oxidant-sensitive probe dichlorodihydrofluorescein diacetate, which was inhibited by pretreatment with indomethacin or by SOD + catalase. We conclude that in porcine coronary microvessels, COX-derived ROS contribute to AA-induced vasodilation.