The epithelial Ca2+ channel TRPV5 serves as a gatekeeper for active Ca2+ reabsorption in the distal convoluted tubule and connecting tubule of the kidney. WNK4, a protein serine/threonine kinase with gene mutations that cause familial hyperkalemic hypertension (FHH), including a subtype with hypercalciuria, is also localized in the distal tubule of the nephron. To understand the role of WNK4 in modulation of Ca2+ reabsorption, we evaluated the effect of WNK4 on TRPV5-mediated Ca2+ transport in Xenopus laevis oocytes. Coexpression of TRPV5 with WNK4 resulted in a twofold increase in TRPV5-mediated Ca2+ uptake. The increase in Ca2+ uptake was due to the increase in surface expression of TRPV5. When the thiazide-sensitive Na+-Cl- cotransporter NCC was coexpressed, the effect of WNK4 on TRPV5 was weakened by NCC in a dose-dependent manner. Although the WNK4 disease-causing mutants E562K, D564A, Q565E, and R1185C retained their ability to upregulate TRPV5, the blocking effect of NCC was further strengthened when wild-type WNK4 was replaced by the Q565E mutant, which causes FHH with hypercalciuria. We conclude that WNK4 positively regulates TRPV5-mediated Ca2+ transport and that the inhibitory effect of NCC on this process may be involved in the pathogenesis of hypercalciuria of FHH caused by gene mutation in WNK4. Copyright © 2007 the American Physiological Society.