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Spea multiplicata (New Mexico spadefoot toad) larvae were exposed to 60, 110, and 1000 mu g L-1 perchlorate dissolved in natural surface water to determine risks associated with perchlorate exposure in desert-adapted anurans. Hind- and forelimb development and tail resorption were measured to identify effects of perchlorate exposure. No perchlorate-related effects on snout-vent length, hindlimb length, and proportion metamorphosed were observed in the highest treatment group (positive control; 1000 mu g L-1) suggesting that either S. multiplicata are not sensitive to the effects of perchlorate at the concentrations tested or that unidentified constituents of natural surface water mitigated perchlorate toxicity. To identify whether surface water mitigated perchlorate toxicity, Xenopus laevis were exposed to 20 and 60 mu g L-1 perchlorate in surface water and synthetic laboratory prepared water (i.e., FETAX media). X laevis exposed to perchlorate dissolved in surface water exhibited no perchlorate-related anti-metamorphic effects, whereas X laevis exposed to perchlorate in FETAX media experienced changes in percent metamorphosing (p < 0.001), time to metamorphosis (p < 0.001), snout-vent length (p < 0.001), and hindlimb length (p < 0.001) as compared to FETAX controls. These results suggest that natural surface water can mediate perchlorate effects at concentrations up to 60 mu g L-1 for X laevis and greater than I mg L-1 for S. multiplicata, potentially due to physicochemical properties of surface water. Capsule: This manuscript discusses the effects of perchlorate in natural surface water to S. multiplicata and X. laevis. (C) 2009 Elsevier Ltd. All rights reserved.