The distribution of endothelin (ET)-like peptide immunoreactivity and mRNA in the nervous system of Aplysia californica was investigated by means of immunocytochemistry and in situ hybridization. Using specific antisera to human endothelin-1 (ET-1) and to the c-terminal peptide of human big endothelin-1 (22-38) (big ET-1), ET-like immunoreactivity was localized to various types of neurons. Immunoreactive nerve fibers for both antisera were prominent throughout the nervous system. Hybridization of radiolabeled complementary RNA probes for human ET-1 or rat endothelin (endothelin-3, ET-3) to paraformaldehyde-fixed, paraffin-embedded sections of various ganglia revealed the presence of a large number of labeled neurons. The number of labeled neurons was higher than those that were immunoreactive for the ET antisera. Preabsorption of the ET antisera with their respective synthetic peptide or hybridization of sections with the sense probes confirmed the specificity of the data. The present study shows that members of the endothelin family may be found in very distant phylogenetic organisms. This conservation suggests a possible role for this peptide as a neurotransmitter and/or neuromodulator in the central nervous systems of diverse species.