The aim of the present study was to assess the effects of ammonia on physiological responses in Korean rockfish (Sebastes schlegeli). Interestingly, no mortality were observed when the specimens ($301.1{pm}8.0g$) were exposed to five levels of un-ionized ammonia ($NH_3$) (control, 1, 2, 4, $8mg;L^{-1}$) for 3 hours. Furthermore, a significantly higher increase in gill $Na^+/K^+$-ATPase (NKA) pump activity with was detected due to the ammonia exposure. The activity of the fishes were found to be 4 and $8mg;L^{-1}$ $NH_3$, which was significantly high compared to normal $1mg;L^{-1}$ $NH_3$. Although ammonia exposure had no effect on plasma $Cl^-$, exposure to both 4 and $8mg;L^{-1}$ $NH_3$, however it led to increase in the plasma $Na^+$, $K^+$ and osmolality levels. Also, prolong ammonia exposure cause increase of plasma cortisol and glucose levels. The increase in glucose was accompanied by an increase in cortisol. The fish exposed to 4 and $8mg;L^{-1}$ $NH_3$ showed significantly higher hematocrit than control group than those exposed to 1 and $2mg;L^{-1}$ $NH_3$. The intensity of cell damage increased with the increase concentration and exposure to ammonia. Furthermore, hyperplasia, separation and epithelial necrosis were also observed in gill tissues. Taken together, the results showed that direction of changes to the investigated parameters can be used to determine the physiological responses of Korean rockfish to ammonia.