Elevated copeptin, a stable surrogate of vasopressin, is recognized to be higher in T1D and predicts diabetic kidney disease and cardiovascular (CV) mortality. Vasopressin increases renal oxygen consumption through enhanced sodium reabsorption and may result in renal hypoxia. Given the natriuretic and cardiorenal protective effects of SGLT2 inhibition (SGLT2i) in clinical trials, our aim was to: 1) examine serum copeptin in adults with T1D after SGLT2i with empagliflozin (EMPA) for 8-weeks; and 2) determine the relationship between copeptin and metabolic, renal and system hemodynamic parameters. In this post hoc, exploratory analysis, serum copeptin concentration, glomerular filtration rate (GFR) by inulin clearance, effective renal plasma flow (ERPF) by PAH clearance, vascular stiffness parameters, plasma RAAS markers, HbA1c, 24-hour urine volume and sodium and glucose excretion were measured in 40 participants with T1D (24.1±5.0 years, HbA1c 8.0±0.9%) during eu- and hyperglycemia at baseline and after 25 mg of daily EMPA. Copeptin concentrations increased under both eu- and hyperglycemic conditions (4.0±2.1 to 5.1±6.0 pmol/L, p=0.005 and p<0.0001, respectively) in response to EMPA. Higher copeptin concentrations were associated with higher HbA1c (β±SE: 0.97±0.30, p=0.003), lower 24h urine volume (β±SE: -3.08±1.05, p=0.006) and fractional sodium excretion (β±SE: -8.30±2.73, p=0.004), after correcting for age, sex, systolic blood pressure (SBP), and HbA1c. There were no significant relationships between copeptin and RAAS markers, vascular stiffness, renal or systemic hemodynamic parameters in fully adjusted models. SGLT2i increased serum copeptin concentrations, which positively correlated with HbA1c and inversely with urine output and natriuresis, but not with renal or system hemodynamic parameters. Further work is required to evaluate the clinical implications of elevated copeptin with SGLT2i; whether it is simply a marker of diuresis or may contribute to CV disease long-term. Disclosure Y. Lytvyn: None. P. Bjornstad: Advisory Panel; Self; Horizon, XORTX. Consultant; Self; Bayer US, Boehringer Ingelheim International GmbH, Bristol-Myers Squibb Company. A. Katz: None. S.K. Singh: None. L.C. Godoy: None. L.T. Chung: None. C.L. Vinovskis: None. B.A. Perkins: Advisory Panel; Self; Abbott, Boehringer Ingelheim International GmbH, Boehringer Ingelheim International GmbH, Insulet Corporation. Research Support; Self; Boehringer Ingelheim International GmbH. Other Relationship; Self; Abbott, Boehringer Ingelheim International GmbH, Lilly Diabetes, Medtronic, Novo Nordisk Inc., Sanofi. D. Cherney: Other Relationship; Self; AbbVie Inc., AstraZeneca, Bayer AG, Boehringer Ingelheim International GmbH, Janssen Pharmaceuticals, Inc., Merck & Co., Inc., Mitsubishi Tanabe Pharma Corporation, Prometic Life Sciences Inc., Sanofi.