In freshwater fishes, ammonia is thought to traverse the gill epithelium almost exclusively via passive diffusion of ammonia as a gas in solution (Randall and Wright, 1987; Ip et al., 2001), so that water NH3 concentration (partial pressure) determines the potential for toxicity. In seawater fish, gill permeability to the ammonium ion may also contribute to toxicity (Randall and Tsui, 2002).