To study the effects of hemodialysis on erythrocyte membrane Na+-K+ ATPase activity, we measured cytochemical Na+-K+ ATPase activity of erythrocyte in 36 patients with chronic uremia. The results were summarized as follows: 1) The Na+-K+ ATPase activity in the predialytic subjects was lower than that of normal controls(P=0), however value in the post-dialytic subjects was greater than that of normal controls(P<0.05). Also the value of postdialytic subjects increased significantly than that of predialytic subjects(P<0.001). 2) When an erythrocyte membrane taken from normal human was incubated with supernatantes of boiled plasma from predialytic individuals, the Na+-K+ ATPase activity of erythrocyte membrane was significantly lower than that of normal control(P<0.01). However, not significant difference between normal control and postdialytic subjects was seen. 3) The percent inhibition of Na+-K+ ATPase in the predialytic individuals was significantly higher than that of postdialytic individuals; the median value of inhibition in the predialytic group was 52.4%(range; -15.5~77.5%) and that of postdialytic group was -15.5%(range; -130.9~38.4%) (P<0.001). Based on the above findings, it is suggested that the hemodialyiss may acutely affect the erythrocyte sodium pump, possibly through the modulation of a dialyzable ouabain-like factor triggered by the extracellular volume change and removal of uremic toxin.