Abstract: The changes of hemoglobin content (HBC) and dissolved oxygen level (DOL) in the blood of juvenile Japanese flounder (Paralichthys olivaceus) were investigated during 12 h exposure in the controlled laboratory experiment at 170.5℃ and under the condition where the rearing water pH (pH 8.0) adjusted abruptly to the experimental water pH of 6.0, 8.0 (control) and 9.0. In spite of the fluctuation of the HBC which occurred during the 12 h exposure, the variance analysis showed that an abrupt change in water pH towards the acidic or alkaline range had no significant influence on HBC of the fish (P0.05). After 4 h exposure, the abrupt change of water pH from pH 8.0 to acidic condition (pH 6.0) and to alkaline condition (pH 9.0) happened with a significant decrease of DOL in the blood (P0.01). However, the DOL in the blood of the test fish exposed to water pH of 6.0 and 9.0 increased as the exposure time increased from 4 h to 12 h. The results indicated that the DOL in the blood was more sensitive to the change of water pH than to the HBC for juvenile P.olivaceus.