Juvenile Paralichthys olivaceus (mean length 7.29 ± 0.59 cm, mean weight 2.41 ± 0.35 g) were exposed to several concentrations of ammonia (0, 6.25, 12.5, 25, 50, and 100 mg/L), nitrite (0, 50, 100, 200, 400, and 800 mg/L), and nitrate (0, 250, 500, 1000, 2000, and 4000 mg/L) for 96 h in 20-L glass tanks. Lethal concentration 50% (LC50) was determined after removing and counting dead fish at 0, 3, 6, 12, 24, 48, 72, and 96 h of exposure. Exposure was significantly toxic to P. olivaceus, and LC50 at 96 h was 26.008 mg/L for ammonia, 768.078 mg/L for nitrite, and 1431.343 mg/L for nitrate. The toxicity profile found for P. olivaceus juveniles was ammonia > nitrite > nitrate. For antioxidant activity analysis such as superoxide dismutase (SOD) and catalase (CAT) activity, liver and kidney tissues were dissected after 96 h of exposure. In liver and kidney tissues, SOD activity was significantly increased at 25 mg/L of ammonia, above 400 mg/L of nitrite, and at 1000 mg/L of nitrate. At these concentrations, CAT activity also increased, except in the kidney, where no change in CAT activity was detected under exposure to nitrate. The results of this study suggest that exposure to nitrogenous compounds such as ammonia, nitrite, and nitrate can induce significant toxicity and alterations in the antioxidant responses of P. olivaceus.