This experiment was performed to investigate the changes in metabolic rate (MO2), critical oxygen saturation (Scrit) and blood chemistry of olive flounder, Paralichthys olivaceus exposed to progressive hypoxia and return to normoxic water at 20℃. The normoxic standard metabolic rate (SMR) and routine metabolic rate (RMR) were 69.5~83.9 and 70.2~156.4 mg O2 kg-1hr-1 with fish biomass, respectively. Scrit was 31.0% DO at 20℃. When returned to 70% DO following exposure to hypoxia (20% DO), the MO2 of the flounder increased twice as much than normoxic SMR, and then decreased in the range of RMR with time. Blood PO2 and plasma lactate after exposure to hypoxia (20% DO) significantly decreased and increased with reducing ambient oxygen saturation, respectively. The levels of cortisol increased with reducing ambient oxygen saturation, but the levels rapidly decreased in the range of normoxic control when returned to ambient water of 70% DO. Plasma glucose levels increased with return to normoxic water after exposure to progressive hypoxia.