The distal convoluted tubule (DCT) reabsorbs ~7% of the filtered sodium load and contributes to blood pressure regulation. Sodium entry here is through the thiazide-sensitive Na-Cl co-transporter (NCC). Both aldosterone and glucocorticoids (GC) can activate NCC. However, we find that the GC-deactivating enzyme, 11b-hydroxysteroid dehydrogenase (11bHSD2), rarely colocalises with NCC1, leaving the transporter vulnerable to activation by GC.NCC has a diurnal rhythm of activation, which is attenuated by chronic GC treatment2. Here, we investigate the role of the mineralocorticoid receptor (MR) and GC receptor (GR) in NCC activation and rhythmicity. Male C57BL6 mice were implanted with pellets containing no drug, spironolactone (50 mg, chronic MR blockade) or RU486 (60 mg, chronic GR blockade). After 5 days, corticosterone (cort, 6 mg/kg, s.c.) or vehicle (veh, 2% DMSO, s.c.) was injected at 8am (nadir of endogenous corticosterone rhythm). 4 hrs later kidneys were taken. In a separate cohort treated as above with RU486 or blank pellets, kidneys were taken every 6 hrs starting at 7am local time (ZT0). NCC threonine phosphorylation (on T53, pT53-NCC) was measured by western analysis as an index of transporter activation. Data are mean±SD. In control mice, acute cort doubled pT53-NCC abundance (1.0±0.49 AU (veh) vs 1.9±0.28 AU (cort), p=0.004, Student’s t test, n=6). MR blockade reduced total NCC protein (0.39±0.21 AU (spironolactone; n=5) vs 0.80±0.18 AU (control; n=6), p=0.004, by post hoc Tukey tests) but acute cort still substantially increased pT53-NCC abundance (1.0±0.46 AU (veh; n=5) vs 3.7±1.4 AU (cort; n=7), p=0.0018, Student’s t test). GR blockade did not affect total NCC abundance (0.59±0.15 AU (RU486) vs 0.80±0.18 AU (control) p=0.15, Student’s t test, n=6 in each group) but did prevent cort-induced phosphorylation (1.0±0.19 AU (veh) vs 1.0±0.15 (cort), p=0.51, Student’s t-test n=6 in each group). GR, MR and NCC mRNA transcript levels were unchanged by either acute cort treatment or chronic RU486 or spironolactone treatment. Sgk1 and per1 transcripts were increased by cort treatment in control mice (0.36±0.07 versus 0.80±0.34 AU, p=0.0029, post hoc Sidak tests, n=5-6); spironolactone and RU486 blocked this. Chronic RU486 treatment dampened the diurnal rhythm of pT53-NCC with elevated levels at ZT0 and ZT 6 (Treatment p=0.049, time p=0.0004, interaction p= 0.16, twoway ANOVA, n=5-6). Total NCC protein was unchanged at any time point. These data suggest MR activation is required for maintenance of tonic levels of NCC protein but GC can regulate NCC function by acutely increasing NCC phosphorylation through GR activation. The data further suggest that GR is required for the NCC’s diurnal rhythm. This has implications for blood pressure control particularly in individuals with altered GC levels, such in Cushing’s disease, chronic stress or corticosteroid therapy.