Objective : β -endorphin, most potent endogenous opioid peptide, is known to play an important role in many homeostatic systems such as the maintenance of blood pressure, regulation of body temperature and the control of pituitary hormone secretion. Previous reports of plasma levels of β -endorphin in patients with chronic renal failure have mostly shown elevated levels. Rut the effect of hemodialysis on the plasma levels of β -endorphin in patients with maintenance hemodialysis is controversial. The aim of this study was to evaluate the effect of a hemodialysis session on the plasma concentrations of β -endorphin in patients with end-stage renal disease and also to investigate changes of hemodynamic response according to the changes of plasma levels of β -endarphin. Methods: We study group comprised 36 patients who had received regular hemodialysis. Blood for analysis of β -endorphin was sampled before and immediately after hemodialysis and measured by immunoradiometric assay. Extracellular fluid / total body water (ECF/TBW) was measured before and after the hemodialysis session by multifrequency bioimpedance analyzer (InBody 2.0®, Biospace Co., Ltd., Seoul, Korea). Systolic and diastolic blood pressure were measured by Centrysystem 3 BP monitor every 30 minutes. Results: 1) As a whole, the predialysis β -endorphin did not differ significantly from postdialysis levels. Blood pressure increased significantly during dialysis. The postdialysis value of ECF/TBW was significantly decreased(0.37±0.02 vs. 0.34±0.02, p<0.01). 2) The patients were divided into 2 groups according to the pre-, and post-dialysis β -endorphin levels(Group 1, predialysis β -endorphin > postdialysis β -endor- phin(n=23); group 2, predialysis β -endorphin ≤ postdialysis β -endorphin(n=13)). 3) During dialysis, the systolic and diastolic blood pressure increased significantly in group 1(p<0.05), but not in group 2. 4) The postdialysis value of ECF/TBW was significantly decreased from baseline value to reference range (0.34±0.01) in group 1, but to above the reference range in group 2. 5) The plasma concentrations of β -endorphin did not change by administration of recombinant human erythropoietin. Conclusion : In conclusion, the elevation in plasma β -endorphin concentrations probably occur to balance the changes in vasocnnstrictive substances. An increase in vasoconstrictive substances is mainly secondary to the decrease in plasma volume during hemodialysis. The data also suggest that certain vasoactive substances might participate in the hemodynamic response to hemodialysis although their exact roles remain to be further elucidated.