The hypothesis was tested that vardenafil, a PDE5 inhibitor, specifically enhances coronary vasodilation during acidosis. In isolated constant pressure perfused guinea pig hearts, infusion of vardenafil (≤ 1 μM) increased coronary flow concentration-dependently 34 % above baseline. In parallel, cGMP release increased (0.44 ± 0.094 vs. 0.14 ± 0.017 pmol/min·g at 0.5 μM vardenafil vs. baseline). Flow increases occurred in the absence of changes in heart function (LVP, heart rate, dP/dtmax, heart rate — pressure product). Infusion of the NO synthase blocker L-NMMA (100 μM) caused a rightward shift of the dose-response curve of vardenafil. To test whether vardenafil treatment may enhance metabolic coronary vasodilation, arterial pCO2 was raised from 38 to 61mmHg, which resulted in a steady state flow increase of 18.8 ± 4.5%. Infusion of vardenafil, given at a threshold flow enhancing concentration, doubled the coronary flow response during hypercapnic acidosis to 38.4 ± 4.2 % (p = 0.004). This flow amplification during acidosis was not shared by the KATP channel opener cromakalim, indicating a specific effect of vardenafil on flow control during myocardial acidosis. We conclude that vardenafil specifically relaxes coronary resistance vessels through NO/cGMP-dependent pathways and increases the coronary flow response toward hypercapnic acidosis. This finding further supports the importance of the NO-cGMP axis in mediation of this flow response.