Short-term treatments (3h) designed to change the relative NH3 (ΔPNHNH3) and NH4+ (ΔNH4+) gradients and sodium transport (JNa) across the gills were employed to analyse the normal mechanism(s) of branchial ammonia excretion (JAmmnet) in trout acclimated to fresh water of pH ≃ 8.0. Control JAmmnet occurred in the absence of, or against, an apparent ΔPNH3 gradient, while ΔNH4+ was positive. Severe acid exposure (pH = 4.06) raised ΔPNH3 and ΔNH4+, abolished JNain, and reduced JAmmnet by 28%, while moderate acidity (pH = 6.64), which also elevated ΔPNH3 had no significant influence on JNain and JAmmnet. Severe alkaline exposure (pH = 9.54) raised ΔNH4+, reduced ΔPNH3 to a very negative value, and decreased JNain and JAmmnet by equimolar amounts, representing 55% and 80% of control levels respectively. Moderate alkalinity (pH = 8.69) had similar effects on ΔPNH3 and ΔNH4+, but reduced JNain and JAmmnet by only 25%. The sodium transport inhibitor amiloride (10−4moll−1 in the external water, pH ≃ 8.0) had very similar effects to pH = 4.06 on both JNain and JAmmnet but did not alter ΔPNH3 or ΔNH4+. The results discount the quantitative importance of NH4+ diffusion and favour a flexible combination of NH3 diffusion and Na+/NH4+ exchange as the major mechanisms of (JAmmnet). with the latter dominating under the particular control conditions of the present study.

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