Four distinct isoforms of voltage-gated sodium channels have been partially cloned in Hirudo medicinalis and cell-specific expression patterns have been established at the level of mRNA (Blackshaw et al., 2003). Isoform-specific antibodies are not currently available for leech. In order to study localisation of voltage-gated sodium channel protein in the leech central nervous system (CNS) we have used commercially available pan-specific voltage-gated antibodies (anti-SP19, Alomone labs, Sigma). These antibodies recognise a highly conserved intracellular epitope (TEEQKKYYNAMKKLGSKK) present in all mammalian isoforms of the alpha subunit of voltage-gated sodium channels (Vassilev et al., 1988). The SP19 epitope is also highly conserved across phyla and has proved anti-SP19 antibodies a useful tool for the study of voltage-gated sodium channels in several invertebrate species including cockroach, blowfly, grasshopper and moth (Gordon et al., 1990). Alignment of the SP19 peptide sequence with each of the leech voltage-gated sodium channel amino acid sequences allowed us to predict that anti-SP19 antibodies would recognise all four isoforms of the channel. We used fluorescent immunohistochemistry with the anti-SP19 antibodies to label voltage-gated sodium channel protein in paraformaldehyde-fixed sections of adult leech CNS. The antibodies labelled the neuronal cell bodies of the segmental ganglia both at the cell membrane and in the soma, suggesting that there is a large cytoplasmic pool of voltage-gated sodium channel in leech neurons. Axonal processes entering the neuropile of the ganglion were also stained and neuropile processes were densely labelled. The connectives between ganglia were immunoreactive and the longitudinal tracts containing bundles of axons were stained in a punctate manner. This observation is consistent with the clustering of voltage-gated sodium channels previously demonstrated in the non-myelinated axons of the mollusc Aplysia (Johnston et al., 1996). The internal capsule of the ganglion and the outer sheath of the ganglion and connectives were not labelled. Pre-incubation of the anti-SP19 antibodies with the SP19 peptide antigen blocked binding of the antibodies to the sections. In conclusion, this is the first application of anti-SP19 antibodies to the study of the leech voltage gated sodium channels and will allow further experiments on the localisation of channels in the embryonic and regenerating leech CNS.