The nucleic acid sequence of a 370 bp cDNA encoding the KTX 2 precursor (a 38 amino acid residues peptide purified from the North African scorpion Androctonus australis Hector and acting as a high affinity blocker of K + channel) was obtained by using PCR amplification (Laraba-Djebari et al., 1994). During this work, oligonucleotide primers designed according to a highly conserved sequence found in all the toxins active on K + channel were used. This allowed the amplification and cloning of new cDNAs, encoding unknown sequences. A peptide has been chimically synthesized according to these sequence informations, and its activity tested by competition experiments on rat brain channel synaptosomes against different channel ligands, i.e. radioiodinated KTX and apamin. The synthetic peptide was able to compete with both KTX and apamin for their specific binding sites. From the genomic DNA of Androctonus australis, the gene corresponding to this cDNA was amplified. A 64 bp intron was found. However, this gene was amplified only from scorpions collected in the area of Beni-Khedache and not from scorpions collected in two other different places.In the South American Tityus serrulatus venom, competition experiments on rat brain synaptosomes against 1 2 5 I-Apamin bound to its receptor site allowed the purification of a new molecule exhibiting a high affinity (0.3 nM) for the small conductance calcium activated K + channel. Its amino acid sequence showed that it is a 36 amino acid residues peptide, close to KTX. Its cDNA was cloned and sequenced. 3D-models of these new tools were designed to find a conformational explanation to their activities.