The effects of formalin on mixed function oxygenase (MFO) system and stress-response were investigated in olive flounder (Paralichthys olivaceus). Olive flounder was exposed to formalin at the concentration of 300 ppm for 1, 2, 4 and 16 h. Levels of stress-response enzymes together with total protein, glucose and osmolality were quantitatively determined in blood, and the activities of phase I (cytochrome P450, ethoxyresorufin deethylase) and phase II (glutathione S-transferase) hepatic enzymes were also determined. Since the formalin-exposure for 16 h resulted no significant changes in aspartate aminotransferase and alanine aminotransferase, specific enzymes for liver damage, it was thought that it did not cause hepatic tissue damage at the concentration of 300 ppm. However, hepatic MFO system was induced at 1 to 4 h, and stress response was induced after 16 h of exposure. Moreover, it is considered that the depression of MFO activity after 16 h of exposure may not be adaptation to formalin, but toxic response. These results suggest that low concentration of formalin does not cause hepatic tissue damage of fish, but could induce MFO and stress response.