This is an experimental and clinical study to investigate the mechanism of the Disequilibrium-Syndrome in hemodialysis from the point of biochemical changes of the body fluid and osmotic pressure. METHOD. Mongrel dogs were used for this study. Three hour hemodialysis using Kolff's single coil kidney was performed on uremic dogs, which had been nephrec tomized bilatelally and, clinically hemodialysis was performed on 40 patients with renal failure, since November 1967, to December 1970. Dialyzers used in the study were either Kolff-Twin-coil type or Kiil's type. Effort has been made in this study to investigate correlations of parameters followed to patient with or without Disequilibrium-Syndrome. 1. Dialyzer. 2. BUN-Dialisanse. 3. Time of dialysis. 4. Severity of patient. RESULT. 1. The level of Urea-N in lymph and serum, resumed to normal by the end of the first hour of dialysis, but those in cerebrospinal fluid decreased very slowly. There were notable changes of excretory gradient on Urea-N between blood and cerebrospinal fluid. 2. Biochemical changes of thoracic duct lymph during hemodialysis were very similar to those of serum. 3. There was slight change of osmotic pressure between serum and cerebrospinal fluid before and after dialysis. 4. None of patients developed Disequilibrium-Syndrome in case of BUN below 110mg/dl, within 3 hour of dialysis, and 30 % of BUN-dializanse. CONCLUSION. It was fond in this study that osmotic-gradient per se might be one, not all, of cause of Disequilibrium-Syndrome. However. in clinical practice, rapid alteratons of homeostasis seems to be related to this syndrome.