The pharmacological properties of onitin, onitinoside and onitisin, three phenolic pterosins, isolated from Onychium siliculosum have been investigated. Their predominant effects are to inhibit the contraction of isolated guinea-pig ileum. In these three compounds,onitin is the most potent one. Onitin (1 x 10 -4M) inhibits both the Dreceptor and M receptor of 5-HT. The antagonism of onitin against histamine is non-competitive.