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Several species of Prangos are traditionally used as emollient, carminative, tonic, anti-flatulent, anthelmintic and anti-thrombotic agents. Osthole, a coumarin isolated from Prangos, are believed to be responsible for some of its effects. In this research, relaxant effects of Prangos ferulacea extract and osthole on rat uterus contraction induced by KCl, acetylcholine (ACh), oxytocin and electrical field stimulation (EFS) was investigated and compared with atropine and salbutamol. P. ferulacea acetonic extract concentration-dependently relaxed uterine contraction induced by KCl (IC(50)=13 ± 0.81 μg/ml), ACh (IC(50)=12 ± 1.38 μg/ml), oxytocin (IC(50)=16 ± 3.14 μg/ml) and EFS (IC(50)=11 ± 1.5 μg/ml). However, the extract at lower concentration (2.5 μg/ml) potentiated the EFS response. Osthole only had inhibitory effect on rat uterus and its relaxant effect was observed at lower concentration in comparison with P. ferulacea extract. Osthole in a similar way inhibited the response to KCl (IC(50)=4 ± 0.13 μg/ml), ACh (IC(50)=4 ± 0.8 μg/ml), oxytocin (IC(50)=4 ± 0.8 μg/ml) and EFS (IC(50)=1.5 ± 0.5 μg/ml). Our results demonstrated that osthole acted directly on uterus smooth muscle to induce relaxation, whereas P. ferulacea caused both contraction and relaxation of rat uterine smooth muscle. The relaxation of osthole might be mediated through Ca(2+) channel blocking activity as it inhibited the response to KCl. Mechanisms other than Ca(2+) channel blocking appeared to be responsible for ACh relaxation effect of osthole.