The pH of plasma is midway between the pK of the [Formula: see text] and [Formula: see text] reactions. Carbon dioxide excretion across the gill acidifies the water, as CO2 hydrates to form [Formula: see text] and H+ ions. The hydration reaction is catalyzed by carbonic anhydrase in the water boundary layer next to the gills and on the gill epithelium. Acidification of the boundary layer is greater than in the bulk flow, particularly in water of high pH and low buffering capacity. Ammonia excretion will raise water pH but this effect is usually masked by the much larger CO2 excretion. At low water pH, below the pK′ of the CO2 hydration reaction, little [Formula: see text] and H+ will be formed, therefore CO2 will have little effect on downstream water pH. Under these conditions, NH3 excretion will alkalinize water as it passes over the gills. Formation of an acid gill water boundary layer as a result of CO2 excretion will trap NH3 as [Formula: see text] and enhance ammonia transfer. Removal of the acid boundary layer, either by increasing the buffering capacity of the water or by reducing CO2 excretion, decreases ammonia transfer in the isolated head of rainbow trout. Water pH can have a marked effect on CO2 and NH3 gradients across the gills. In alkaline waters, the rise in blood NH3 levels will result in a marked increase in body stores of total ammonia. Tilapia (Oreochromis alcalicus grahami) in Lake Magadi, Kenya, at a water pH of 10, excrete nitrogenous wastes as urea, to avoid the problem of the buildup of toxic levels of ammonia when the fish are exposed to these alkaline conditions.