Effects of the coastal oceanographic conditions on sexual maturation, salinity tolerance, and expression of vasotocin (VT) gene in homing chum salmon, Oncorhynchus keta, were studied by four years of fieldwork and transfer experiments. In fieldwork in 1992 and 1993, fish were sampled at three sites along their migratory pathway on the Sanriku coast, Japan. In transfer experiments in 1994 and 1995, fish captured in the seawater (SW) environment were transferred to SW or freshwater (FW) aquaria and sampled 1 to 4 days later. The distribution of cold and warm currents, which governs the oceanographic conditions of the Sanriku coast, were deduced from the mean sea surface temperature. Maturity of homing fish was estimated by gonadal states. Salinity tolerance was estimated by plasma Na+ levels and mortality in SW environment. Changes in VT gene expression were assessed by a quantitative dot blot analysis of the hypothalamic levels of VT mRNA. Homing fish were fully mature, and showed high plasma Na+ levels and high mortality in the SW environment in 1992 and 1994 when the warm current dominated. In the 1994 transfer experiment, VT mRNA levels markedly increased in the SW-retained males, whereas the levels were decreased by FW-transfer in both sexes. Homing fish were not fully matured in 1993 and 1995 when the branch of cold current reached the Sanriku coast. In the 1995 transfer experiment, VT mRNA levels did not change in both FW-retained and FW-transferred fish. In conclusion, the warmer oceanographic conditions affected the maturity and salinity tolerance in homing salmon, which in turn altered the osmotically-induced expression pattern of VT gene.