The residue of Thr-5 in mu -conotoxin GIIIA (GIILA), a receptor site I sodium channel blocker, was replaced with Cys, The synthesized [Cys(5)]GIIIA had a similar 3D structure to the native GIIIA, revealed by CD and NMR, [Cys(5)]GIIIA and its tagged peptides inhibited the electrically stimulated contraction of the rat diaphragm with relatively comparable potency to that of GIIIA. Since the contractile response to electrical stimuli is caused by the activation of sodium channels, [Cys(])(5)GIIIA could be a prototype for synthesizing useful tools for the analysis of sodium channels. Thus, [Cys(5)]GIIIA could be a prototype for synthesizing useful tools for the analysis of sodium channels.