Pituitary hormones regulate various physiological functions during spawning migration in salmonid. Cytological features of pituitary cells were therefore immunocytochemically examined by use of antisera against homologous hormones in pre-spawning chum salmon (Oncorhynchus keta) caught from Ishikari Bay and the Chitose River in October, 1996. Immunoreactivity and sizes of pituitary cells were determined by a computer-aided image analyzing technique. Immunoreactivity in growth hormone (GH) cells was stronger with enlarged cell sizes in freshwater (FW) fish than in seawater (SW) ones of both sexes. Majority of prolactin (PRL) cells also had significantly stronger immunoreactivity with enlarged cell sizes in FW fish than in SW animals of both sexes. Immunoreactivity in somatolactin (SL) cells was markedly stronger with enlarged cell sizes in FW fish than in SW ones of both males and females. In addition, greater portions of SL cells were strongly stained in FW animals than in SW ones. A greater portion of gonadotropin (GTH) I cells had stronger immunoreactivity with reduced cell sizes in FW fish of both sexes, when compared with SW fish. Conversely, GTH II cells had significantly stronger immunoreactivity with enlarged cell sizes in FW ones of both sexes. In proopiomelanocortin (POMC)-derived hormone producing cells, adrenocorticotropin (ACTH) cells had stronger immunoreactivity in FW animals of both sexes, while cell sizes did not change. In melanotropes, the cells immunoreactive to alpha-melanophore stimulating hormone (alpha-MSH) antiserum had stronger immunoreactivity with reduced cell sizes only in FW males, while the cells immunoreactive to beta-endorphin antiserum had stronger immunoreactivity with reduced cell sizes in FW fish than in SW animals of both sexes. Implication of these results was discussed along with previous reports on gene expression of pituitary hormone precursors.