Procedures for localizing immunoreactive Met-enkephalin and substance P were applied to the adenohypophysis of larval and adult sea lamprey, Petromyzon marinus. Immunopositive reaction to anti-Met-enkephalin antiserum was found in certain adenohypophysial cells of both larvae and adults. This reaction was specific, as judged by preabsorption of the antiserum with synthetic Met-enkephalin and subsequent elimination of the immunoreaction. An immunopositive response to one of three anti-substance P antisera, also selective for certain adenohypophysial cells was obtained in both larval and adult glands. This reaction was not completely eliminated by preabsorption with authentic substance P. Although the substance P immunoreaction is not specific for authentic mammalian substance P, its localization is characteristic and distribution of cells exhibiting it is described. Immune-staining of Met-enkephalin was seen in most of the cells of the rostral pars distalis, and it was as strong or stronger in larvae (ammocoetes) of Petromyzon as it was in adults. The substance P response was found in about half of the cells of the caudal pars distalis and in a few cells of the rostral pars distalis. In ammocoetes substance P staining was limited to a few cells of the caudal pars distalis, and no cells in the rostral pars distalis were stained. Thus, Met-enkephalin staining in Petromyzon is a general property of cells in the rostral pars distalis both in larvae and adults and cannot be correlated with such events in the life cycle as metamorphosis or reproduction. On the other hand, cells containing materials reactive with the anti-substance P serum appear at a very low level in the larva and grow in number and intensity of staining in a time pattern that parallels development of reproductive capacity. Furthermore, the disparity in time of synthesis and adenohypophysial and cellular distribution of Met-enkephalin-like and substance P-like materials indicates that these two peptides are not functionally related, as has been suggested for some higher vertebrates.