Intradialytic hypotension (IDH) is related to high morbidity and mortality. There is evidence that arginine-vasopressin (AVP) responses could play a role. Copeptin is a reliable biomarker of AVP. In this study, copeptin, aldosterone, epinephrine, and norepinephrine levels in patients with IDH were evaluated throughout a hemodialysis (HD) session and compared with the control group. The study includes 15 patients who were normotensive during HD and 15 patients with IDH with a minimum HD vintage of 1 year. Blood samples were collected before the initiation of an HD session (T-0), in the mid-session for control group, 30 min after mean arterial pressure drop for IDH patients (T-1), and at the end of the session (T-2). Groups had similar demographic features and health parameters, interdialytic weight gains, and ultrafiltration amounts. The IDH group had a mean arterial pressure decline of 39.9 (+/- 6.4) mm Hg. Copeptin levels of the control group increased an average of 79.9 (+/- 97.5) pmol/L at T-1 and an additional 24.8 (+/- 33.9) pmol/L at T-2. In the IDH group, copeptin level increases at T-1 and T-2 were 3.2 (+/- 5.5) pmol/L and 34 (+/- 44.6) pmol/L, respectively(.) Copeptin levels of the IDH group were significantly lower at T-1 (P < 0.001) and at the T-0-T-2 interval than control group (P = 0.05). In the control group, aldosterone levels distinctly decreased, and in the IDH group, aldosterone levels were elevated (P < 0.001). Small changes were detected in epinephrine and norepinephrine levels for both groups but did not reach significance (P = 0.6 and P = 0.3, respectively). Lower copeptin level alterations suggest inadequate AVP responses in patients with IDH.