In order to study cation transport in vivo we have measured the changes in plasma and intra-erythrocytic rubidium concentrations following an oral load of rubidium chloride. The changes in plasma rubidium concentration are related to the distribution of rubidium to all the body tissues and the changes in intra-erythrocytic rubidium concentrations provide an example of rubidium uptake by one particular tissue. In eight healthy volunteers pretreatment with a loading dose of digoxin (20 micrograms/kg) enhanced the rise in plasma rubidium concentrations and attenuated the rise in intra-erythrocytic rubidium concentrations after the oral load of rubidium chloride. Ten patients with chronic renal failure, compared with a well-matched control group, were found to have changes similar to, but more marked than, those caused by digoxin, i.e. a much greater rise in plasma rubidium concentrations and a much smaller rise in intra-erythrocytic rubidium concentrations, after the oral load of rubidium chloride. These findings are consistent with wide-spread reduction in Na+, K+-ATPase activity in subjects who have taken a loading dose of digoxin and patients with chronic renal failure. They are, therefore, consistent with the findings of previous studies in vitro and show that it is possible to demonstrate changes in cation transport in vivo.