We examined the influence of speciation on the kinetics of silver uptake and deputation in the gills of two freshwater fish, the rainbow trout (Oncorhynchus mykiss) which has high branchial Na+ and Cl- uptake rates and is relatively sensitive to silver, and the European eel (Anguilla anguilla, yellow stage) which has low ion uptake rates and is relatively resistant to silver. Fish previously acclimated to the appropriate chloride level were exposed to (110m) AgNO3 (1.3 mug 1(-1), sublethal) for 24 h in synthetic softwater with either low (10 muM) or high (1200 muM) chloride concentration, and then followed over a subsequent 67-day post-exposure period in silver-free water of the same chloride content. The exposures were therefore mainly to the free ion, Ag+ in the low chloride water versus mainly to the neutral aqueous complex, AgClaq in the high chloride water. In trout, but not in eel. water chloride is known to protect against physiological disturbances and toxicity caused by Ag+. In both fish species, at both chloride levels, silver uptake exhibited complex kinetics. Gill silver loading occurred slowly until 6 h, then rose greatly to a peak at 12 h, followed by significant net depuration thereafter during continued exposure. By 24 h, net gill loading was three-to fivefold greater from AgClaq than from Ag+ exposure in both species, and threefold greater in trout than in eel under both conditions, with trout holding a lower fraction of the whole body burden in their gills, During the post-exposure period, depuration of silver from the gills occurred rapidly in trout, but very slowly in eel, such that gill silver burdens were greater in eel throughout the 67-day period on both an absolute and relative basis (e.g. 35% of whole body burden in eel versus < 3% in trout at day 8). The kinetics of depuration were described by two phase exponential models, with break points between the fast and slow phases at I and 15 days for trout and eel, respectively. We conclude that speciation affects not only uptake rates but also the kinetics of depuration. When silver is loaded from AgClaq it is clearly more labile than from Ag+ exposures, with 1.6-1.8-fold greater loss rates during the fast phases in both species. Differences in branchial silver uptake between eel and trout correlate well with differences in acute toxicity, but are not as large as differences in ion uptake rates. The complex time-dependent patterns of gill loading. and the higher loading from AgClaq than from Ag+, mean that gill total silver burden is not an appropriate endpoint for biotic ligand modelling. (C) 2002 Elsevier Science B.V. All rights reserved.