The aquaporins are a diverse family of proteins with homologues expressed in most classes of organism. As well showing permeability to H2O, a sub-group – the aquaglyceroporins – also allow passage of small solutes such as glycerol and urea across biological membranes. The European Eel (Anguilla Anguilla), an euryhaline teleost, is able to adapt to survive in a wide range of salinities, and changes in the expression pattern of eAQP3 play a role in this adaptation process (Lignot et al.,2002,). In a previous study we have shown that when expressed in Xenopus oocytes, eAQP3 increased urea flux, but did show to be permeable to H2O (Walton et al., 2008). A recent study suggests that the permeability of eAQP3 to H2O is sensitive to extracellular pH (pHo) (MacIver et al., 2009). In the current study we have investigated the sensitivity of urea transport by eAQP3 to pHo. Oocytes were isolated from humanely killed Xenopus laevis, and injected with 5ng (0.1ng/nl) cRNA encoding eAQP3 or 50nl of H2O. The oocytes were maintained at 18C and experiments were performed at room temperature on day 3 or 4 following injection. Uptake of urea was performed as described previously (Fenton et al., 2000). Osmotic water permeability (Pf) was calculated from the rate of volume change upon exposing the oocytes to a hypotonic solution. Statistical analysis was performed using one-way ANOVA coupled with Newman-Keuls post hoc comparison test. Significance assumed at the 5% level. The results of the study are summarized in Table 1. As expected an increase in pHo stimulated Pf in oocytes expressing eAQP3. A similar trend was observed for eAQP3-dependent urea transport. Urea flux was increased compared to control oocytes at pHo 7.5 and 8.2, with maximum uptake occurring at pHo 8.2. In the eel, AQP3 is expressed in the gill and intestine. Studies in MDCK cells indicate it targets to both the apical and basolateral membranes (Walton et al., 2009), providing a potential route for transepithelial solute movement. Plasma pH lies close to pH8.2 and so transport of water and urea through eAQP3 across the basolateral membrane will be optimal. The transport across the apical membrane will depend on the ambient pH (pH 6.5 – 7.9 for freshwater and pH 7.9 – 8.3 for seawater).