Type 4 Bartter syndrome (BS) is caused by genetic mutations in barttin, which is coded for by BSND. Barttin serves as the beta-subunit of the CIC-K chloride (Cl-) channel, which is widely expressed in distal nephrons. Type 4 BS is characterized by severely impaired reabsorption of salt, which may cause polyuria, hypokalemia, and metabolic alkalosis. Calcineurin inhibitors reportedly induce renal salt retention and hyperkalemia by enhancing the phosphorylation of the sodium (Na+)-potassium (K+)-2Cl(-) cotransporter (NKCC2) and Na+-Cl- cotransporter (NCC). In addition, we have previously reported that tacrolimus, a calcineurin inhibitor, increases the levels of phosphorylated NCC. In this study, we administered tacrolimus to barttin hypomorphic (Bsnd(neo/neo)) mice, a murine model of type 4 BS that exhibits polyuria, hypokalemia, and metabolic alkalosis. Administration of tacrolimus increased the serum K+ level and suppressed urinary K+ excretion. Furthermore, after treatment with tacrolimus, Bsnd(neo/neo) mice increased levels of phosphorylated NCC and NKCC2. We conclude that tacrolimus partially improves clinical phenotypes of Bsnd(neo/neo) mice, and that calcineurin inhibitors might be effective for treating type 4 BS. (C) 2019 Elsevier Inc. All rights reserved.