The influence of dietary potassium on the retention of chronically, ingested Cs137 in the rat was investigated. The levels of potassium in the diet were either 0.2%, 0.6% or 1.8%. Radiocesium was added to the drinking water and the daily Cs137 intake determined. At 1, 3, 7, 14 and 35 days, groups of 5 rats were killed and the Cs137 content of muscle, liver, testes, femur, whole blood and brain determined. At 35 days, a 9-fold elevation in dietary potassium caused a 2-fold reduction in Cs137 retention. This is probably the maximum effect. Tissue potassium levels were not altered by dietary potassium levels. At 1 day, the levels of Cs137, in decreasing order, were liver, muscle, testes, femur, whole blood, brain. At 35 days, the levels, in decreasing order, were: muscles liver, testes, brain, femur, whole blood. The Cs/K ratio of the tissues divided by the Cs/K ratio of diet varied almost proportionally with die-tarypotassium levels; a 9-fold increase in dietary potassium caused about a 5-fold increase in Cs/k ratio (diet[forward arrow] tissue). Therefore, it appears that the concept of "discrimination factor" has severe limitations in theoretical and practical application for this ion pair. A short-term chronic ingestion study, using K42 and Cs137, was also undertaken. High levels of potassium in the diet decreased K42 retention but did not influence Cs137 retention. It was concluded that potassium does not effectively compete with cesium for membrane transport sites or intra-cellular binding sites.