Arachnoid venoms contain numerous peptides with ion channel modifying and cytolytic activities. We developed a green fluorescent protein (GFP)-based assay that can monitor the changes in currents through overexpressed inwardly rectifying K(+) channels (Kir2.1), in which GFP expression was increased by blockade of Kir2.1 current. Using this assay, we screened venom of many spider species. A peptide causing GFP decreasing effect was purified and sequenced. Electrophysiological and pain-inducing effects of the peptide were analyzed with whole-cell patch-clamp recordings and hot-plate test, respectively. Among venoms we screened, soluble venom from Lachesana sp. decreased the GFP expression. Purification and sequencing of the peptide showed that the peptide is identical to a pore-forming peptide purified from Lachesana tarabaevi venom. Whole cell patch-clamp recordings revealed that the peptide had no effect on Kir2.1 current. Instead, it induced a current that was attributable to the pore-formation of the peptide. The peptide was selectively incorporated into hyperpolarized, i.e., Kir2.1 expressing, cells and for this reason the peptide decreased GFP expression in our Kir2.1 assay. The pore-formation positively shifted the reversal potential and induced burst firings in the hippocampal neurons in a synaptic current-independent way. The application of the Lachesana sp. peptide induced pain-related behavior in mice. The peptide, which was found in Lachesana sp. venom, formed pores and thereby depolarized neurons and induced pain. Our data suggested an additional physiological role of the pore-forming peptides.