Extracts from aerial parts of Sweet grass 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. The effect of the 5,8-dihydroxybenzopyranone on smooth muscles tone of small mesenteric blood vessels was investigated. METHODS. Contractility of isolated small mesenteric blood vessels (350–450 mm in diameter) from adult male guinea pigs was investigated by small blood vessels wire myography. RESULTS. Coumarins from Sweet grass significantly suppressed the vascular contractility to 80 mM of potassium chloride and to phenylephrine (30 mM) in dose-dependent mode. Concentration of coumarins 0.01 mg/ml decreased the contraction force of blood vessels to phenylephrine until 41.16±13.47 per cent (p<0.01; n=5), but didn't inhibit endothelium-dependent relaxation to acetylcholine (p>0,05) and didn't influence augmented contraction after nitric oxide syntase inhibition (p>0,05). Relaxation to sodium nitroprusside (100 mM) was improved by coumarin, reducing the contraction from 24.8±2.61% before the incubation till 14.96±4.44% after the incubation (p<0,05). CONCLUSION. Coumarins of Sweet grass inhibit vascular smooth muscle contractility without suppressing of endothelium-dependent and endothelium-independent relaxation.