Physiological mechanisms of silver toxicity (as silver nitrate) to the sensitive rainbow trout (Oncorhynchus mykiss) (96 h LC50: 10.2 mu g silver l(-1), in soft, low chloride water) and the more tolerant European eel (Anguilla anguilla)(96 h LC50: 34.4 mu g silver l(-1), in the same water) were investigated during acute exposure to silver, using concentrations varying from 3 to 22 mu g silver l(-1). Silver was present either predominantly in the form of ionic silver, or in the form of silver chloride complexes (AgClaq). Inhibition of the branchial Na+,K+-ATPase enzyme activity and the active influx of Na+ leading to net Na+ loss were the key toxic effect in both species. In the rainbow trout, but not in the European eel, Cl- influx was also impaired during silver exposure. However, even under control conditions, Cl- influx was negligible in the eel. Water Cl- clearly protected against the silver-induced physiological disturbance in rainbow trout, presumably by changing the speciation of silver from ionic silver to AgCl complexes. However, such a protective effect-was not observed in the European eel. Differences in whole body Na+ turnover rates between the two species (1.1% per day in the European eel versus 19%, per day in the rainbow trout) together with the lack of effect of silver exposure on Cl- homeostasis in the European eel are hypothesized to be the main reasons for the different silver tolerance observed in the two species. (C) 2000 Elsevier Science B.V. All rights reserved.