Recent studies have shown that gain-of-function mutations of WNKs (With No Lysine Kinases) are associated with salt-sensitive hypertension in human patients. The physiological functions of WNKs in the regulation of body fluid homeostasis, however, are still poorly understood. We therefore investigated the intrarenal localization and regulation of WNK-1 and WNK-4 by osmolality and changes in salt intake. Using RT-PCR in rat microdissected nephron segments, WNK-1 mRNA was found along the whole nephron with most prominent expression in the glomerulus, cTAL, DCT and CCD, whereas expression of WNK-4 mRNA was mainly confined to the DCT and CCD. Hyperosmolality (900mOsm NaCl) increased WNK-1 and WNK-4 mRNA significantly in rIMCD cells by 1.4-fold and 2.5-fold, respectively. A significant upregulation WNK-1 and WNK-4 mRNA expression was also detected in kidneys of rats kept for 6 weeks on a high salt diet (8% NaCl), and was 2.9-fold and 1.6-fold for WNK-1 and WNK-4 mRNA, respectively. These results demonstrate that at the transcription level WNK-1 and WNK-4 are markedly expressed in the aldosterone-sensitive, distal nephron and are sensitive to osmotic stress and changes and salt intake. These results support a physiological role of WNKs in the maintenance of body-fluid homeostasis.