Radioimmunoassay and chromatography analyses of hypothalamic luteinizing hormone-releasing hormone (LHRH) have demonstrated the presence of LHRH-like immunoreactive peptides in a wide range of vertebrates. Contrary to previous reports, the molecule differs in various vertebrates. Avian, reptilian, and teleostean LHRH's are chemically distinct from the mammalian peptide but are in themselves indistinguishable. However, amphibian LHRH appears to be identical to the mammalian peptide. These findings have interesting evolutionary implications.