Mutations in the WNK1 gene, encoding a serine-threonine kinase of the WNK (With No lysine (K)) family, have been implicated in two rare human diseases, Familial Hyperkalemic Hypertension (FHHt) and Hereditary Sensory and Autonomic Neuropathy type 2 (HSAN2). Alternative promoters give rise to a ubiquitous isoform, L-WNK1, and a kidney-specific isoform, KS-WNK1. Several other isoforms are generated through alternative splicing of exons 9, 11 and 12 but their precise tissue distribution is not known. Two additional exons, 8b and HSN2, involved in HSAN2, are thought to be specifically expressed in the nervous system. The purpose of this study was to establish an exhaustive description of all WNK1 isoforms and to quantify their relative level of expression in a panel of human and mouse tissues and in mouse nephron segments. For the latter purpose, we developed a new methodology allowing the determination of the proportions of the different isoforms generated by alternative splicing. Our results evidenced a striking tissue-specific distribution of the different isoforms and the unexpected presence of exon HSN2 in many tissues other than the nervous system. We also found exon 26 to be alternatively spliced in human and identified two new exons, 26a and 26b, within intron 26, specifically expressed in nervous tissues both in humans and mice. WNK1 should therefore no longer be designated as a 28- but as a 32-exon gene, with 8 of them - 8b, HSN2, 9, 11, 12, 26, 26a and 26b - alternatively spliced in a tissue-specific manner. These tissue-specific isoforms must be considered when studying the different roles of this ubiquitous kinase.