Aquatic Toxicology j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a q u a t o x Differential responses in ammonia excretion, sodium fluxes and gill permeability explain different sensitivities to acute high environmental ammonia in three freshwater teleosts a b s t r a c t We examined the acute physiological responses to high environmental ammonia (HEA), particularly the linkages between branchial ammonia fluxes and unidirectional Na + fluxes, as well as urea excretion, cortisol, and indicators of gill permeability in three freshwater teleosts differing in their sensitivities to ammonia; the highly sensitive salmonid Oncorhynchus mykiss (rainbow trout), the less sensitive cyprinid Cyprinus carpio (common carp) and the highly resistant cyprinid Carassius auratus (goldfish). Fish were acutely exposed to two sub-lethal ammonia concentrations (as NH 4 HCO 3) at pH 7.9: 1 mM for a period of 12 h, identical for all species, and 5 mM for the cyprinids and 1.4 mM for the trout for 3 h. Elevation of plasma cortisol at both levels of HEA was apparent in all species. At 1 mM, ammonia excretion (J amm) was inhibited to a greater extent in trout than cyprinids and concurrently a significantly higher plasma ammonia level was evident in trout. However J amm was reversed in all species at 5 or 1.4 mM. Goldfish showed a significant increase in urea excretion rate (J urea) during HEA exposure. In carp and trout, neither level of HEA elevated J urea but urea production was increased as evidenced by a considerable elevation of plasma urea. At 1 mM HEA, Na + imbalance became progressively more severe in trout and carp due to a stimulation of unidirectional Na + efflux (J Na out) without a concomitant increase in unidirectional Na + influx (J Na in). Additionally, a transient reduction of J Na in was evident in trout. Goldfish showed an opposite trend for J Na out with reduced efflux rates and a positive Na + balance during the first few hours of HEA. However, after 12 h of exposure, both J Na in and J Na out were also increased in both carp and goldfish, whereas only J Na out was increased in trout, leading to a net Na + loss. Na + homeostasis was entirely disrupted in all three species when subjected to the 5 or 1.4 mM ammonia for 3 h: J Na in was significantly inhibited while considerable activation of J Na out was observed. Diffusive water efflux rates and net K + loss rates across the gills were enhanced during HEA only in trout, indicating an increment in gill transcellular permeability. Transepithelial potential was increased in all the species during ammonia exposure, but to the least extent in goldfish. Overall, for several different physiological systems, trout were most disturbed, and goldfish were least disturbed by HEA, helping to explain the differential ammonia tolerance of the three species.