The European Eel is a euryhaline teleost, which is able to adapt to survive in a fresh or seawater environment. The aquaporin water channels play important roles in osmoregulation in both mammals and fish. Recently it was shown that seawater adaptation leads to changes in the expression levels of the aquaporins eAQP1 and eAQP3 (Lignot et al.,2002, Martinez et al., 2005). Screening of eel cDNA libraries highlighted two putative clones with high homology to mammalian AQP8, which we have labelled eAQP8 and eAQP8b. In the current study we have investigated the permeability properties of the eel aquaporins, eAQP1, 3, 8 and 8b. The cDNA encoding the eel proteins eAQP1, eAQP3, eAQP8 and eAQP8b was subcloned into the Xenopus expression vector pTLN (Lorenz et al., 1996) and cRNA prepared. Oocytes were isolated from humanely killed Xenopus laevis, and injected with 5ng (0.1ng/nl) cRNA encoding the appropriate aquaporin or 50nl of H₂O. 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. Methylammonium (MA) uptake was performed in a fashion similar to that for urea uptake, with the oocytes exposed to a solution containing 20µM MA chloride supplemented with 1µCi/ml 14C MA chloride for 15 minutes. Statistical analysis was performed using unpaired t-tests or one way ANOVA coupled with Dunnett’s test. Statistical significance has been assumed at the 5% level. The results are summarised in Table 1. We saw a significant increase in Pf in oocytes expressing eAQP1, 8 and 8b, indicating that these proteins act as water channels. eAQP3 and 8 caused significant increases in urea permeability. In addition oocytes expressing eAQP1 and eAQP3 exhibited a significant increase in MA permeability compared to control, suggesting that eAQP1 and eAQP3 also provide a pathway for NH₃ across the cell membrane. The aquaporins fall into two classes the ‘classical’ aquaporins and aquaglyceroporins. Our results suggest that eAQP1 and eAQP8b have permeability properties similar to the classical aquaporins, while eAQP3 and eAQP8 fit into the aquaglyceroporin class.