Radioimmunoassay (RIA), radioreceptor assay and chromatography were used to study the occurrence of calcitonin gene-related peptide (CGRP) in a primitive teleost, the eel, Anguilla anguilla. Immunologically and biologically active CGRP-like molecules were found in brain, heart, kidney, liver, spleen and ultimobranchial body with the higher concentrations in brain, spleen and heart. Gel exclusion chromatography of heart and spleen extracts followed by SDS-PAGE showed that the eel CGRP-like molecules presented a molecular weight between 3.30 and 3.95 kDa similar to that of human CGRP. The wide distribution of CGRP reflects its multiple role as brain neuromediator and peripheral paracrine effector as described in mammals. In comparison, the distribution of calcitonin (CT) was much more restricted, immunologically and biologically active CT-like molecules being localised in the ultimobranchial bodies (UBB) that is the site of CT synthesis in non-mammalian vertebrates. In plasma, CGRP-like concentrations were 10 to 100 higher than those of CT. These high concentrations in a primitive teleost strengthen the possible endocrine role of CGRP in early vertebrates and emphasise the important role of this hormone in evolution.