Net ammonia fluxes (JAmm) were measured in adult freshwater rainbow trout in vivo under a variety of conditions designed to inhibit unidirectional sodium uptake (JinNa; low external [NaCl], 10(-4) mol l-1 amiloride), alter transbranchial PNH3 and NH4+ gradients [24 h continuous (NH4)2SO4 infusion, or exposure to 1 mmol l-1 external total ammonia at pH 8] and prevent gill boundary layer acidification (5 mmol l-1 Hepes buffer). Inhibition of JinNa with amiloride or low external [NaCl] under normal conditions reduced JAmm by about 20 %, but did not prevent the net excretion of ammonia during exposure to high concentrations of external ammonia. Increasing the buffer capacity of the ventilatory water with Hepes buffer (pH 8) reduced JAmm by 36 % and abolished the effect of amiloride on ammonia excretion. No evidence could be found to support a directly coupled apical Na+/NH4+ exchange. We suggest that any dependence of ammonia excretion on sodium uptake is caused by alteration of transbranchial PNH3 gradients within the gill microenvironment secondary to changes in net H+ excretion. Under normal conditions (pH 8, low external ammonia) gill boundary layer acidification facilitates over one-third of the total ammonia excretion. During exposure to high concentrations of external ammonia in poorly buffered water, estimates of transbranchial PNH3 gradients from measurements of bulk water pH and total ammonia concentration (TAmm) may be grossly in error because of boundary layer acidification. Prevention of boundary layer acidification with Hepes buffer during exposure to high cocncentrations of external ammonia revealed that the local transbranchial PNH3 gradient at the gill may in fact be positive (blood to water), negating the need for an active NH4+ transport mechanism. In freshwater trout, NH3 diffusion may account for all ammonia excretion under all experimental conditions used in the present study.