Leukocyte cation content and sodium pump transport were measured in 14 undialysed uraemic and 15 CAPD patients and compared with results from healthy controls. Sodium content was elevated in the undialysed group compared with the CAPD group (P = 0.05) or the healthy controls (P less than 0.001), but this abnormality could not be clearly attributed to increased sodium influx or reduced sodium pumping. Sodium content correlated with plasma urea (rs = 0.69, P = 0.005) and creatinine (rs = 0.56, P = 0.031) concentrations in the CAPD group only. In subgroups of 12 undialysed and ten CAPD patients, erythrocyte sodium content and pump transport were measured at the same time as the leukocyte studies. Erythrocyte sodium was less in the undialysed group (P less than 0.01) and again there was no evidence of significant sodium-pump inhibition. The abnormalities in sodium content in undialysed patients were therefore in opposite directions in the two cell types, and were significantly less in both cell types in CAPD patients. These results emphasise the dangers of assuming that abnormalities of sodium transport seen in a single cell type can be taken as evidence of a generalised abnormality.