Hydrogen sulfide (H2S) has emerged as a new and important member in the group of gaseous signalling molecules. However, the molecular transport mechanism has not yet been identified. Prediction of its actual membrane permeability, PM, according to Overton's rule (1) is hampered by the fact that the partition coefficient into the organic phase is not known. Because of structural similarities with H2O, it was hypothesized that aquaporins may facilitate H2S transport across cell membranes. We tested this hypothesis by reconstituting the archeal aquaporin AfAQP from sulfide reducing bacteria Archaeoglobus fulgidus into planar membranes and by monitoring the resulting facilitation of osmotic water flow and H2S flux. To measure H2O and H2S fluxes, respectively, sodium ion dilution and buffer acidification by proton release were recorded in the immediate membrane vicinity. Both [Na+] and pH were measured by scanning ion selective microelectrodes. A lower limit of PM,H2S > 0.5 ± 0.4 cm/s was calculated by numerically solving the complete system of differential reaction diffusion equations and fitting the theoretical pH distribution to experimental pH profiles. Even though reconstitution of AfAQP significantly increased water permeability through planar lipid bilayers, PM,H2S remained unchanged. The fact that cholesterol and sphingomyelin reconstitution did not turn these membranes into a H2S barrier indicates that H2S transport through epithelial barriers, endothelial barriers and membrane rafts also occurs by simple diffusion and does not require facilitation by membrane channels (2).1. Missner, A & Pohl, P (2009) 110 years of the Meyer-Overton rule: Predicting membrane permeability of gases and other small compounds ChemPhysChem 10:1405-1414.2.Mathai, JC, Missner, A, Kugler, P, Saparov, SM, Zeidel, ML, Lee, JK, Pohl, P (2009) No facilitator required for membrane transport of hydrogen sulfide Proc.Natl. Acad. Sci. USA 106:16633-16638.