Immunohistochemical studies have indicated that glucagon-containing cells are present in the intestinal mucosa of the agnathan Petromyzon marinus (sea lamprey) but are absent from the pancreas. Glucagon was isolated from an extract of intestinal tissue taken from specimens of sea lamprey during their parasitic phase. The primary structure of the peptide was established as: His-Ser-Glu-Gly-Thr5-Phe-Thr-Ser-Asp-Tyr10-Ser-Lys-Tyr-Leu- Glu15-Asn-Lys-Gln-Ala-Lys20-Asp-Phe-Val-Arg-Trp25-Leu- Met-Asn-Ala. This amino acid sequence shows 8 substitutions compared with that of mammalian glucagon but, with the exception of the COOH-terminal alanine residue, lamprey gut glucagon contains no structural features that have not been previously seen in glucagons isolated from the pancreata of gnathostomes. The amino acid sequence of lamprey glucagon-like peptide (GLP) demonstrates that the primary structure of this peptide has been less well conserved than that of glucagon. The sequence His-Ala-Asp-Gly-Thr5-Phe-Thr-Asn-Asp-Met10-Thr-Ser-Tyr- Leu-Asp15-Ala-Lys-Ala-Ala-Arg20-Asp-Phe-Val-Ser-Trp25- Leu-Ala-Arg-Ser-Asp30- Lys-Ser shows 16 amino acid substitutions compared with the corresponding region of mammalian GLP-1 and 15 substitutions compared with that of salmon GLP.