Considering blood-sucking habits of leeches from surviving strategy of view, it can behypothesized that leech saliva has analgesia or anesthesia functions for leeches to stayundetected by the host. However, no specific substance with analgesic function hasbeen reported from leech saliva although clinical applications strongly indicated thatleech therapy produces a strong and long lasting pain-reducing effect. Herein, a novelfamily of small peptides (HSTXs) including 11 members which show low similarity withknown peptides was identified from salivary glands of the leech Haemadipsa sylvestris.A typical HSTX is composed of 22–25 amino acid residues including four half-cysteines,forming two intra-molecular disulfide bridges, and an amidated C-terminus. HSTX-Iexerts significant analgesic function by specifically inhibiting voltage-gated sodium (Na V )channels (Na V 1.8 and Na V 1.9) which contribute to action potential electrogenesis inneurons and potential targets to develop analgesics. This study reveals that sodiumchannel inhibitors are analgesic substances in the leech. HSTXs are excellent candidatesor templates for development of analgesics.