For the first time in a marine teleost (the long-horned sculpin; Myoxocephalus octodecimspinosus), the maintenance of blood pH, PCo2> [HCO3~] and the net movements of NH4 , HCO3~ and H + between the fish and the water have been studied during exposure to ammonia stress induced either by infusion (NH4C1 or NH4HCO3; 5 mmol kg" ) or by external application (NH4C1; approx. 1 mmol I" ). Following NH4CI infusion, a rapid decrease in blood pH (0'36units) and [HCO3-] (2-38 mmol I") was observed, and within l h about 40% of the ammonia load had been excreted to the water. Analysis of NH4 + and HCO3~ transfers revealed that the total ammonia (TAmm) efflux was due to a loss of NH3 and NH4 + in approximately equal proportions when an outwardly directed NH3 diffusion gradient was established. Infusion of NH4HCO3 induced only small changes in plasma pH, and the rate of net HCO3~ excretion was some 90% higher than that of NH4 + over 20 h. These data indicate a predominance of NH3 as the form of ammonia lost. In both infusion experiments, a presumed intracellular buffering of a majority of the ammonia load was noted. High external TAmm induced an initial uptake of NH4 , but after 4 h of exposure ammonia efflux resumed even though NH3 diffusion gradients were negligible. Thus, in this seawater teleost, a role for the excretion of ammonia in the form of NH4 + is also likely.