The endocrine cells and nerves in the islet and the gut of the arctic lamprey Lampetra japonica were examined immunocytochemically by using antisera against brain-gut peptides and amine. The cellular composition of the islets as reported by previous researchers in European species of the lamprey was confirmed in the present study. The islet consisted exclusively of insulin immunoreactive cells in the larvae (ammocoetes), whereas in the adult somatostatin immunoreactive cells were added to the insulin immunoreactive cells; the gut epithelium in the adult was now devoid of somatostatin cells. In the gut of the lamprey, the endocrine cells--which were flask-shaped with a cytoplasmic process extended to the lumen--were classified into three types in the larvae, but were represented by a single type in the adult. In the larval lamprey, the first type was immunoreactive for somatostatin, the second one for gastrin/cholecystokinin (CCK) and the third cell type was immunoreactive for glucagon, pancreatic polypeptide and FMRFamide, simultaneously. In the gut of the adult lamprey, the single type of endocrine cell reacted simultaneously to C-terminal specific anti-glucagon serum, N-terminal specific anti-glucagon serum, anti-bovine PP serum, anti-neuropeptide Y serum and anti-FMRFamide serum. These cells occurred most frequently in the upper intestine, their distribution decreasing from the middle to the lower intestine. Two types of peptide containing nerves were identified in the islet and the gut of the larval and adult lamprey. The first type of neurons (perikarya and fibers) was immunoreactive for serotonin and calcitonin gene-related peptide (CGRP), and was located in the mucous and muscular layer of the intestine and in the islet. The second type of neurons contained both serotonin- and gastrin releasing peptide (GRP)-like immunoreactivities and was scattered exclusively in the muscular layer of the gut. In larval and adult lampreys, a few serotonin/CGRP immunoreactive nerve cell bodies and beaded fibers were found in the connective tissue around the islet cell cords. These nerve fibers were sometimes closely apposed to the blood capillaries and to the islet cells. These findings indicate that a neuroendocrine correlation comparable with that in mammals may have been established in the islet of this most primitive vertebrate.