Extract from aerial parts of Sweet grass (Hierochloe odorata) are expected to become a source of natural antioxidants for food industry — it contains the new identified coumarin derivative 5,8-dihydroxybenzopyranone, possessing radical scavenging activity (1). No data on the effects of this compound on living organs are available. The effect of the extract fraction, containing 5,8-dihydroxybenzopyranone on smooth muscles tone of blood vessels was investigated. Adult male guinea pigs, weighting 400-550 g were humanely killed under halothane anaesthesia by cervical transection and exsanguination. Contractility of isolated small mesenteric blood vessels (300-500 μm in diameter) was investigated by small blood vessel wire myography (2). Three different concentrations (1.0 mg/ml, 0.1 mg/ml and 0.01 mg/ml) of Sweet grass extract fraction were added to the organ bath for 30 min of incubation. Contraction of blood vessels was induced by addition of K+-rich solution (80 mM) or α1-receptor agonist phenylephrine (30 μM) before and after the incubation. The endothelium-dependent relaxation was investigated by acetylcholine (1 nM—10 μM) after the precontraction with phenylephrine (30 μM). Results were expressed as mean±SEM and evaluated using t test for paired data. Vascular contractility was supressed by the application of extract fraction in a dose-dependent manner (Fig.1). Incubation in 0.1 mg/ml of the extract significantly decreased the force of contraction induced by K+-rich solution and phenylephrine until 37.42±13.23 % (p<0.001; n=8) and until 11.82±6.11 % (p<0.001; n=5), respectively. With an extract concentration of 0.01 mg/ml the contraction force induced by phenylephrine was decreased significantly (p0.05; n=8). No changes in endothelium-dependent relaxation were revealed after incubation at 0.01 mg/ml (n=5, p>0.05). Inhibition of NOS by L-NAME (100 μM) had no influence on the decrease of contractility (p>0.05; n=5). Sweet grass (Hierochloe odorata) extract fraction, containing coumarol derivative 5,8-dihydroxybenzopyranone, inhibits vascular contractility by the direct action on smooth muscles but not on the endothelium.