Introduction: Hypertension (HT) affects 1 in 4 age 25+ years, contributing to ~12.8% of global deaths. Thiazide diuretics are amongst the most widely used Anti-HT drugs that reduce blood pressure (BP) by inhibition of the Na+-Cl- Cotransporter (NCC) in the renal distal convoluted tubule (DCT) causing salt-wasting. Genetic disorders & knockout (KO) mice targeting proteins SPAK, OSR1 or WNKs that form a cascade regulating NCC activity & expression in the DCT have also resulted in BP modulation via parallel changes in NCC function & renal Na+ excretion. So the WNK/SPAK/OSR1 pathway represents a novel therapeutic HT target (1-3). Recently mouse protein-25 alpha (MO25α/CAB39) was discovered to be the master regulator of SPAK/OSR1; it colocalises with NCC, increases SPAK/OSR1 activity up to 100-fold independent of WNKs, facilitates SPAK/OSR1 monomer interactions & stabilises active conformations, which in turn enhance NCC activity & Na+ reabsorption (3-5). It was hypothesised that MO25 deficiency would result in a hypotensive salt-wasting phenotype, however the embryonic lethality of MO25α-KO mice has limited in vivo data & focus to its sister isoform MO25beta (MO25β/CAB39L). MO25β shares 79% sequence identity with MO25α, interacts similarly with substrates such as LBK1 & has high renal expression (4). Thus MO25β-KO mice were utilised to test in vivo effects of the MO25α paralog. Methods: Age matched C57Bl/6 MO25β-KO mice & wildtype (WT) littermates between 2-4 months were either fed a normal Na+ diet (0.25%) or placed on a high Na+ diet (3%) for 10 days before switching to a low Na+ diet (0.03%) with urine collected for electrolyte analysis at 0hrs, 3hrs, 6hrs, 24hrs & 4 days post-switch. BP was measured by direct carotid manometry (MLT844, AD Instruments) under terminal anaesthesia (isoflurane 2% at 2L/min O2). The procedure terminated with blood collection via cardiac puncture for electrolyte analysis & excision of the kidneys. Data are presented as mean ± SEM; means were compared by Student’s t-test with P <0.05 considered statistically significant. Results: There were no differences between KO vs. WT mean arterial pressures on normal Na+ [80.2 ± 2.4 mmHg (n=4) vs. 79.0 ± 2.3 mmHg (n=3)] or low Na+ diets [80.0 ± 1.2 mmHg (n=11) vs. 80.7 ± 1.1 mmHg (n=11)]. No significant differences were detected for urine or plasma levels of creatinine, Na+, K+, Cl-, Ca2+, Mg2+ or PO43- on any diet with the one exception of Mg2+ on a normal Na+ diet (see table). We therefore conclude MO25β does not play a significant role in BP or electrolyte homoeostasis in vivo.