OBJECTIVE Adiponectin, an important adipocytokine secreted by adipocytes, has anti-inflammatory and atheroprotective effects on vascular tissue via the adiponectin receptor (adipoR). However, the action of adiponectin in the retinal microcirculation is unknown. We examined the direct effect and underlying mechanism of the vasomotor action of adiponectin in porcine retinal arterioles. METHODS Porcine retinal arterioles (internal diameter, 60-90 μm) were isolated, cannulated, and pressurized (55 cmH2O) without flow in this in vitro study. Videomicroscopic techniques were used to record changes in diameter in response to adiponectin. RESULTS The retinal arterioles dilated in a dose-dependent (0.125-7.5 μg/mL) manner in response to adiponectin. The vasodilation decreased significantly after removal of the endothelium. N(G)-nitro-L-arginine methyl ester (a nitric oxide [NO] synthase inhibitor), 1H-1,2,4-oxadia-zolo[4,3-a]quinoxalin-1-one (a soluble guanylyl cyclase inhibitor), but not wortmannin (a phosphatidylinositol 3-kinase inhibitor) inhibited the effect of adiponectin-induced vasodilation comparable with that of denudation. Pretreatment with compound C, an activated protein kinase (AMPK) inhibitor, partially but significantly reduced vasodilation. Incubation with GW6471, a peroxisome proliferator-activated receptor blocker, did not significantly inhibit vasodilation by adiponectin. AdipoR1 and adipoR2 immunoreactions were observed in the endothelium of retinal arterioles. CONCLUSIONS Adiponectin elicits mainly endothelium-dependent dilation of the retinal arterioles. Endothelium-dependent vasodilation likely induced by adiponectin results from NO via activation of guanylyl cyclase that is partially dependent on AMPK activity. Understanding the effect of adiponectin on the retinal vasculature may help improve potential therapies for retinal vascular disorders, especially diabetic retinopathy in patients with type 2 diabetes mellitus.