Localization of GABA, tyrosine hydroxylase (TH), NADPH diaphorase, transcription factor Pax6, and the proliferative cell nuclrar antigen (PCNA) in the periventricular area of the brain of the masu salmon Oncorhynchus masou of varying age groups was studied. The presence of heterogeneous cell popula tions with radially orientated outgrowths in the diencephalon, central gray matter of the dorsomedial teg mentum, medulla oblongata, and spinal cord of the masu salmon was shown. TH , GABA immunopositive, and NADPH d positive cells are located in the areas of PCNA immunogenic proliferative zones. It is pos sible that these cells are descendants of the radial neuroglia and that they participate in the growth and phys iological regeneration of diencephalic and medullar structures. In various age groups of the masu salmon, the marking of TH , GABA, PCNA, and Pax6 reveals the neuromeric structure of the brain.