Aquaporin-8 (AQP8)-facilitated transport of NH₃ has been suggested recently from increased NH₃ permeability in Xenopus oocytes and yeast expressing human or rat AQP8. We tested the proposed roles of AQP8-facilitated NH₃ transport in mammalian physiology by comparative phenotype studies in wildtype vs. AQP8 null mice. AQP8-facilitated NH₃ transport was confirmed in mammalian cell cultures expressing rat or mouse AQP8, in which the fluorescence of a pH-sensing yellow fluorescent protein was measured in response to ammonia (NH₃/NH₄⁺) gradients. Relative AQP8 single channel NH₃-to-water permeability was ~0.03. AQP8-facilitated NH₃ and water permeability in a native tissue was confirmed in membrane vesicles isolated from testes of wildtype vs. AQP8 null mice, in which BCECF was used as an intravesicular pH indicator. A series of in vivo studies were done in mice, including: a. serum ammonia measurements before and after ammonia infusion, b. renal ammonia clearance, c. colonic ammonia absorption, and d. liver ammonia accumulation and renal ammonia excretion following acute and chronic ammonia loading. Except for a small reduction in hepatic ammonia accumulation and increase in ammonia excretion in AQP8 null mice loaded with large amounts of ammonia, there were no significant differences in wildtype vs. AQP8 null mice. Our results support the conclusion that AQP8 can facilitate NH₃ transport, but provide evidence against physiologically significant AQP8-facilitated NH₃ transport in mice.