Sphingosine-1-phosphate (S1P) is a biologically active lysophospholipid.Spinster homolog 2 (Spns2) acts as an S 1P transporter in zebrafish and mice regulating heart development and lymphocyte trafficking respectively.The mechanism of action of Spns2 is still elusive in mammals especially considering the multiple roles of S1P signalling.Here, we demonstrate that Spns2-deficient mice rapidly lost auditory sensitivity and endocochlear potential (EP) from 2 to 3 weeks old.Spns2 is expressed in both the organ of Corti and lateral wall of the cochlea.We found progressive degeneration of sensory hair cells in the organ of Corti, but the earliest defect was a decline in the EP suggesting that dysfunction of the lateral wall was the primary lesion.In the lateral wall of adult mutants, we observed structural changes of marginal cell boundaries and of strial capillaries, and reduced expression of several key proteins involved in the generation of EP (Kcnj 10, Kcnq1, Gjb2 and Gjb6), but these changes were likely to be secondary.Permeability of the boundaries of the striavascularis and of the strial capillaries appeared normal.Targeted inactivation of Spns2 in red blood cells, platelets, or lymphatic or vascular endothelial cells did not affect hearing, but targeted ablation of Spns2 in the cochlea using a Sox10-Cre allele produced a similar auditory phenotype to the original mutation.Our study suggests that local Spns2 expression is critical for hearing in mammals.These findings indicate that Spns2 and S 1P signalling are required for normal maintenance of the EP and hence for normal auditory function.