Haemoglobin, methaemoglobin, blood nitrite concentration and acid-base balance were measured in European eel Anguilla anguilla following exposure to 0 (control), 0.142, 0.356, 0.751 and 1.549m m nitrite in fresh water for 24h. Blood GOT (glutamate oxaloacetate transaminase) and GPT (glutamate pyruvate transaminase) activities and whole animal ammonia-N and urea-N excretions were also measured. Blood nitrite, methaemoglobin, P O 2 (oxygen partial pressure), GOT, and whole animal ammonia-N excretion and urea-N excretion increased directly with increasing ambient nitrite concentrations, whereas blood pH, P CO 2 , and HCO-3 were inversely related to ambient nitrite concentration. An accumulation of nitrite in the blood of A. anguilla following 24h exposure to elevated ambient nitrite as low as 0.751m m increased its blood methaemoglobin, P O 2 , GOT and nitrogen excretion, but decreased its P CO 2 (carbon dioxide partial pressure), HCO-3 and functional haemoglobin. Copyright 2002 The Fisheries Society of the British Isles. Published by Elsevier Science Ltd. All rights reserved.