Transferring adult Japanese flounder (Paralichthys olivaceus) directly from salinity 30 into salinity 3 and 30, effects of abrupt low salinity stress on plasma osmolality, cortisol(COR), prolactin(PRL), growth hormone(GH) were studied after 0 h, 8 h, 1 d, 2 d, 5 d, 8 d and 14 d, and the changes of Na⁺, K⁺ and Cl^- in the plasma were also monitored. Results showed that, all plasma indicators had no significant changes between control group and 0h blank group; after transfer to salinity 3, plasma osmolality decreased in 8 h, continued to decline in 1 d, and then rose slightly in 2 d and then kept stable; the levels of Na⁺ and Cl^- showed similar changes, decreased in 8h-1d, then stabilized; K⁺ gradually increased after transferring into salinity 3, then rose to the highest level in 1 d, while in 2 d it dropped to the level that was slightly higher than control group; the concentration of COR in plasma rose to the highest value in 1 d, then declined in 2 d, significantly lower than control group; PRL level reduced firstly then elevated 2 d,and it was significantly higher than the value in 30‰ seawater. Plasma GH in experimental group decreased in 1 d and then remained the lower level. As a euryhaline fish, Japanese flounder has strong adaptability to abrupt low salinity stress,and it could increase or decrease hormones rapidly, such as COR, PRL, GH, to re-establish a new internal environment homeostasis.