1. Sea anemone toxin II (ATX II) which keeps the activated sodium channels open, can be labelled at its histidine residues with 125I up to a specific radioactivity of 500 Ci/mmole. Upon intraventricular injection in mice, ATX II causes acute, short-lasting hyperexcitation and convulsions. Its LD50 in mice is between 25 and 50 ng of the native peptide, and between 50 and 100 ng of the radioactive material per animal. 2. The labelled peptide is bound to mouse diaphragm from where it can be displaced by ATX II and, even better, by scorpion neurotoxin but not by other basic peptides, e.g., histone or aprotinin. Binding is not significantly influenced by 50 mM potassium, by replacing sodium with choline, by veratridine or tetrodotoxin. In contrast to binding of alpha-bungarotoxin, binding of ATX II is not changed by denervation of the diaphragm. ATX II binds not only to the muscular but also to the tendinous moiety of the mouse diaphragm. 3. ATX II lowers the surface tension of water. Further experiments are needed to establish the usefulness of 125I-ATX for labelling sodium channels in excitable membranes.