The in vitro effects of xipamide in a concentration range of 10(-8) to 10(-2) M were investigated on various Na+ and K+ transport systems in human red blood cells. Xipamide inhibited the anion carrier or DIDS-sensitive LiCO3- -influx starting from a concentration of 10(-5) M. However, a decrease in the Na+, K+-pump and the Na+, K+-cotransport activity and a rise in the passive permeability of the cell membrane was only observed starting from a concentration of 10(-4) M xipamide.