Aim: Increased oxidative stress plays important roles in vascular dysfunction in patients undergoing coronary artery bypass graft surgery. Hydrogen peroxide (H₂O₂) is used as an experimental model for oxidative stress. The present study was designed to assess the effects of levosimendan pretreatment on the contractile effects induced by H₂O₂ in human saphenous vein (HSV) segments. Methods: We studied H₂O₂ induced contractions of isolated HSV mounted in standard tissue baths. H₂O₂ (10^-6 – 10^-3 M) was added cumulatively to the organ bath. Concentration-response curves to H₂O₂ were repeated in the presence of levosimendan (10^–8 M). In the second series of experiments, strips were contracted with 5-HT (10^–5 M). When the contraction reached a stable plateau, H₂O₂ was administrated cumulatively into the organ bath. The same procedure was conducted in the presence of levosimendan. Results: Pretreatment of the SV strips with levosimendan significantly reduced the contractile response to each concentration of H₂O₂ . 5-HT produced contractions in SV strips. Further treatment of these strips with H₂O₂ resulted in statistically significant concentration-dependent increases in tension. Preincubation of the tissues with levosimendan did not significantly influence the maximum amplitude of the 5-HT-induced tone but inhibited the contractile effect of H₂O₂ on the 5-HT-induced contraction. Conclusion: Pretreatment of HSV with clinical concentrations of levosimendan inhibits the vasoconstriction caused by oxidative stress, indicating its potential preventive effect against oxidative stress induced graft spasm.