Compared to other organs the cells of the renal inner medulla are challenged with an environment of highly variable osmolality mediated mainly by marked increases in Na+ and urea. High tonicity leads to the activation of the tonicity enhancer binding protein (TonEBP/NFAT5), which induces the expression of several transporters and enzymes. This in turn leads to cellular accumulation of organic osmolytes thereby protecting the cells from high extracellular tonicity. We performed gene expression profiling using primary cultured rat renal inner medullary collecting duct cells which were cultivated for 6 days at 300, 600 or 900 mosmol/kg. Differentially expressed genes were identified using the BRB Array Tools and functional molecular network were identified with Ingenuity Pathway Analysis. Comparing cells grown in 300 vs. 600 mosmol/kg, more than 1600 genes and vs. 900 mosmol/kg more than 3300 genes were differentially expressed. In both cases the ratio between the up and down regulated genes was approximately one. Besides known genes like aquaporin-2, betaine and urea transporters the expression of several other genes were identified for which regulation by tonicity had not been described before. For example pathway analysis showed that the WNT/b-catenin (Ctnnb) signalling was affected. Western blot analysis showed marked differences within the Ctnnb expression and phosphorylation profile. Also the intracellular localization of Ctnnb was altered by osmolality changes. While Ctnnb showed diffuse distribution at 300 mosmol/kg at 600 or 900 mosmol/kg it accumulated at the plasma membrane and in part also in the nucleus. Similar differences were observed for the phosphorylated Ctnnb protein. In conclusion the adaptation to a higher tonicity is associated with massive changes in gene expression. Among these changes in Ctnnb expression and localization was evident indicating a prominent function of this pathway for the osmoadaptation. Supported by Else-Kröner-Fresenius.