WNK4 kinase mutations produce the autosomal dominant disorder familial hyperkalemia and hypertension (FHH), also known as pseudohypoaldosteronism type II, by a molecular mechanism that is not completely understood. In vitro experiments in frog oocytes showed that WNK4 affects ion transport systems such as the Na-Cl cotransporter and the renal outer medullary potassium channel. Some features of FHH suggest that long-term effects are involved in WNK4 signaling. In addition, WNK1 and WNK2, paralogs of WNK4, were shown to be involved in MAP kinase signaling. We therefore investigated possible WNK4 involvement in MAP kinase signaling. We stimulated HEK 293 cells overexpressing WNK4 by hypertonicity or using EGF, and measured phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 and p38. WNK4 augmented the phosphorylation of ERK1/2 and p38 in response to both hypertonicity and EGF. The FHH-producing and kinase-deficient mutants behaved similarly to wild-type WNK4. Hypertonicity stimulation was accompanied by cellular relocalization of WNK4 as manifested by its reversible disappearance from the supernatant fraction following extraction with a detergent-containing buffer. Live-cell microscopy showed that the cytoplasmic-soluble WNK4 redistributes rapidly to membrane-bound organelles, which, in the case of WNK1 kinase, were recently shown to represent trans-Golgi network/recycling endosomes. In contrast, EGF stimulation was not accompanied by redistribution of WNK4 as determined by cell fractionation or cell microscopy. The observation that WNK4-induced MAP kinase stimulation caused by hypertonicity, but not that caused by EGF, is associated with WNK4 subcellular redistribution suggests that this redistribution has a role in WNK4 signaling.