In this paper we report on the peptide maps of the three polypeptides produced by reduction of denatured 19-S thyroglobulin from guinea pig. These maps were obtained by both chemical cleavage (CNBr) and by limited enzymatic proteolysis (Staphylococcus aureus SV 8 protease). Analysis of these peptides showed a strict correspondence among the electrophoretic bands produced by cleavage of the guinea pig thyroglobulin components. These results support the idea that thyroglobulin, even though it has an apparently complex molecular structure, contains polypeptide sequences which are repeated in the elementary chain. Since similar structures should correspond to similar functions, each of these regions may work as a functional domain for the biosynthesis of the thyroid hormones.