The hexane extract of Syzygium samarangense (Ss.Hex) dose-dependently (10D1000 μg/ ml) relaxed the spontaneously contracting isolated rabbit jejunum. Four rare C-methylated flavonoids with a chalcone and a flavanone skeleton were isolated from Ss.Hex and were subsequently tested for spasmolytic activity. All flavonoids, identified as 2′-hydroxy-4′,6′- dimethoxy-3′-methylchalcone (1), 2′,4′-dihydroxy-6′-methoxy-3′,5′-dimethylchalcone (2), 2′,4′-dihydroxy-6′-methoxy-3′-methylchalcone (3), and 7-hydroxy-5-methoxy-6,8-dimethylflavanone (4), showed dose-dependent spasmolytic activity in the rabbit jejunum with IC 50 values of 148.3 ± 69.4, 77.2 ± 43.5, 142.4 ± 58.6 and 178.5 ± 37.5 μg/ml (mean ± SEM), respectively. The dihydrochalcone derivative of compound 1, 2′-hydroxy-4′,6′-dimethoxy-3′- methyldihydrochalcone (5), when tested for spasmolytic activity, did not significantly relax the smooth muscle relative to the other compounds. Verapamil, a standard spasmolytic, has an IC 50 value of 0.16 ± 0.04 μg/ml. This is the first report of the relaxant activity of chalcones, specifically of compounds 1-3.