Effects of O-2 supersaturation on metabolism and growth were studied in juvenile turbot (Scophthalmus maximus L.). When fish were reared for 30 days in water containing O-2 at 147% or 223% air saturation, there were no significant differences in food intake, growth, food conversion or protein utilization compared to fish exposed to normoxia (100% air saturation in water outlet). Exposure to hyperoxia resulted in increased body fat deposition. Daily rates of O-2 consumption of resting fish were not affected by O-2-concentrations, and there were no significant differences in rates of nitrogenous excretion among fish exposed to the different O-2-concentrations. Turbot tolerated severe hyperoxia, 350% air saturation, for 10 days.There were changes in acid-base balance that compensated for the respiratory acidosis resulting from O-2 supersaturation. Blood pH was regulated within 24 h (it averaged 7.69 over the 30-day experiment) by significant increases in plasma CO2 content and pCO(2). Plasma CO2 was dose dependent averaging 11.3 and 18.9 mmol l(-1) under 147% and 224% O-2 saturation, respectively, compared to 6.7 mmol l(-1) under notmoxia. Over the 30-day experiment, the only change in hydromineral balance was a slight, but non-significant decrease in plasma chloride content in fish exposed to hyperoxia (137 mmol l(-1) compared to 139 under normoxia). There were no changes in haematocrit, haemoglobin and red blood cell counts (they averaged 18.3%, 3.7 g dl(-1) and 1.37 x 10(6) mm(-3). respectively) and no signs of stress (plasma cortisol averaged 3.8 ng ml(-1)) related to exposure to O-2-supersaturation for 30 days.