ObjectiveTo test what length is needed for polyamine binding both intrinsic gate and pore docking site to block the cloned strong inwardly rectifying channel (Kir2. 1 channel). Methods The effect of alkylamine analogues (DA 5, DA 8, DA 10 and DA 12) and the competitive interaction of polyamine toxin, philanthotoxin (PhTx), on expressed Kir 2. 1channel in Xenopus oocytes were examined by using giant excised inside-out patch-clamp technique. ResultsThe results showed that along with the increase of the length of DAs ,the value of Kd decreased and the high affinity binding increased gradually. However,PhTx had the strongest effect on interfering the DA10 binding between intrinsic gate and pore docking site and had a less effect on DA12. Conclusion DA10 may be the right length for polyamine to block the channel. And maybe there is a hydrophobic interaction between DA12 and C-terminal domain of this channel, which then stabilize the DA12 binding between these two points and decrease the effect of PhTx on DA12.