Vasotocin (VT) and isotocin (IT) are teleost neurohypophysial hormones produced by neurosecretory neurons in the magnocellular part of preoptic nucleus (PM) of the hypothalamus. Several previous studies indicated that neurohypophysial hormones are involved in teleost reproductive behavior. The changes in the expression of VT and IT genes were thus studied by an in situ hybridization technique and an immunohistochemical avidin-biotin-complex method in pre-spawning chum salmon (Oncorhynchus keta). Male and female fish were caught at Atsuta, the mouth of the Ishikari Bay, and at Chitose, upstream to the Ishikari River in October, 1994. The former and the latter are referred to as bay fish and river fish, respectively. The intensity of autoradiographic hybridization signals were determined in individual neurosecretory neurons in the rostroventral, middle, and dorsocaudal parts of the PM. in the females, the levels of VT and IT mRNAs in the river fish were significantly lower than those in the bay fish in all the three loci in the PM, whereas VT and IT immunoreactivities in the river fish were higher than those in the bay fish. These results suggest that both the rates of transcription and release of VT and IT were decreased in pre-spawning female chum salmon. Contrary, in the males, the levels of IT mRNA and IT immunoreactivity in the river fish were greater than those in the bay fish particularly in the rostroventral part of the PM, whereas conspicuous changes were not seen in the levels of VT mRNA and VT immunoreactivity. The present results thus revealed sexually different expression of neurohypophysial hormone genes in the preoptic nucleus of pre-spawning chum salmon when compared between bay and river fish. The regulation of neurohypophysial hormone gene expression may be different between the male and the female during the last stages of spawning migration.