The long-term objective of our research is to show that internal factors may be key to triggering metamorphosis and directing the life history types in lampreys (parasitism versus nonparasitism). Since neuropeptide Y family peptides are key players in the endocrine-mediated feeding and reproductive events in mammals, a role for these peptides in the control of feeding behavior and development can be predicted for lampreys. We have investigated the expression pattern of these peptides in the brain and in the gut during different stages of the life cycle of the parasitic lamprey, Petromyzon marinus, and the nonparasitic lamprey, Ichthyomyzon gagei. We provide a description of the cloning and sequencing of P. marinus and I. gagei cDNA for neuropeptide Y (NPY), peptide tyrosine-tyrosine (PYY), and peptide methionine-tyrosine (PMY). Using sequence-specific primers, the mRNA expression patterns for these peptides in brain and gut of larval (ammocoete) and adult (juvenile, prespawning) lampreys were examined by semiquantitative RT-PCR. The observations extend a potential role of neuropeptide Y family peptides in the modulation of feeding behavior and reproductive maturity in lampreys.