One of the causes of pseudohyperpotassemia is the induction of potassium (K) release from blood cells by various factors such as cold-storage of blood samples, various drugs including ouabain and so on. We speculated on the effect of storage temperature and therapeutic drugs used on the elevation of K levels in vitro. Dobutamine was selected for in vitro experiment. The therapeutic dose of dobutamine (76 micrograms/l) was mixed with whole blood and incubated at 37 degrees C. A significant increase in K concentration was observed after incubation for 6 hours. These results suggested that many injection drugs used clinically induce K release from blood cells in vitro as well as in vivo and the administration of such drugs could be one of the causes of pseudohyperpotassemia. We developed a simple and practical method to determine the K-flux activity of erythrocytes. Employing this method, hemodialysis patients and aged subjects were evaluated. These were shown to be reduced compared with those of healthy subjects. In 30 hemodialysis patients, K-flux activity and the mean serum K concentration expressed as the average of 4 measurements correlated negatively (r = -0.482, p < 0.01). The K-flux activity of the erythrocytes collected after hemodialysis therapy was increased significantly (p < 0.05) compared with that of erythrocytes collected before therapy. We conclude that the K-flux activity of erythrocytes might participate partly in the regulation of serum K concentration in hemodialysis patients and that the inhibitory factors on K-flux activity might be removed by hemodialysis therapy.