Blood serum nitrite/nitrate concentrations were determined in 10 patients with chronic uraemia during 4-hour hemodialysis (HD). Cuprophane, polysulphone and polyacrylnitrile dialysis membranes were used during HD procedures. Blood serum concentrations of nitrite/nitrate were increased in HD patients prior to hemodialysis as compared with respective values in healthy subjects. During hemodialysis, regardless of the membrane type used, blood serum nitrite/nitrate concentrations were progressively decreasing as compared with initial values found before hemodialysis. The lowest decrease of blood serum nitrite/nitrate concentrations was found during HD with cuprophane membrane.