We examined the effects of acute hypoxia on vascular tone and coronary blood flow (CBF) in rabbit coronary arteries. In the pressurized arterial preparation of small arteries (< 100 µm) and the Langendorff-perfused rabbit hearts, hypoxia induced coronary vasodilation and increased CBF in the presence of glibenclamide (KATP channel blocker, GB), Rp-8-Br-PET-cGMPs (cGMP-dependent protein kinase inhibitor, Rp-cGMPs), and MRS 1334 (adenosine A3 receptor inhibitor); these increases were inhibited by the inward rectifier K+ (Kir) channel inhibitor, Ba2+. These effects were blocked by the adenylyl cyclase inhibitor SQ 22536 and by the cAMP-dependent protein kinase (PKA) inhibitors Rp-8-CPT-cAMPs (Rp-cAMPs) and KT 5720. However, cGMP-dependent protein kinase (PKG) was not involved in the hypoxia-induced increases of the vascular diameter and CBF. In summary, our results suggest that acute hypoxia can induce the opening of Kir channels in coronary artery that has small diameter (< 100 µm) by activating the cAMP and PKA signalling pathway, which could contribute to vasodilation and therefore increased CBF.