Nitrate concentrations accumulate in aquaculture systems from the nitrification of ammonia. In recirculating production systems that have limited fresh water exchange, nitrate levels often are elevated over 200 mg/L. Nitrates are usually considered non-toxic to fishes. Work in our laboratory, however, has indicated that exposure to elevated nitrate concentrations for extended periods of time may cause considerable pathology. Hybrid striped bass exposed to elevated nitrate levels of 200 mg/L (added as sodium nitrate) for 1 month had a significantly reduced humoral immune response. Hematological and serum chemistry changes included a reticulocytosis, a monocytosis, elevated creatinine levels and a severe hypochloremia. Histopathologic examination revealed renal and hepatic failure, and hyperplasia and fusion of gill lamellae. The fish began to die after seven weeks at elevated nitrate levels. Fish exposed to potassium and calcium salts of nitrate, and fish from recirculating systems with elevated nitrates developed similar changes in blood analyses. Collectively, the data indicate that nitrates at high levels may be toxic to hybrid striped bass.