1. The concentration of potassium in the erythrocytes and the plasma of forty-one normal subjects and twenty-five diabetic patients was measured and the results were used to calculate the total amount of potassium in the erythrocyte mass and the total amount of potassium in the plasma. The total body potassium was measured in a whole-body monitor. 2. In normal subjects a close correlation was found between total erythrocyte potassium and total body potassium and also between total plasma potassium and total body potassium. 3. The regression relation between total body potassium and total erythrocyte potassium in normal subjects was used to predict the total body potassium in diabetic patients. There was reasonable agreement between the measured and predicted total body potassium but there was poor agreement between the measured total body potassium and that predicted from the patient's height and age or height, weight and age.

1. By using a whole-body monitoring technique iron metabolism has been investigated in patients suffering from chronic renal failure who required regular dialysis treatment. 2. Oral absorption of inorganic iron was low. 3. The incorporation of radioactive iron into erythrocytes was diminished. 4. The rate of loss of radioactive iron from the body was significantly greater than in normal control and non-dialysed patients with chronic renal failure. 5. Iron exchange between dialysate and patient was studied. Patients with chronic renal failure are known to have a decreased rate of erythropoiesis and to develop abnormalities in iron metabolism (Kaye, 1958; Logue, Lange & Moore, 1958; Boddy, Lawson, Linton & Will, 1970). However, considerable controversy exists about the effect of haemodialysis on iron metabolism in such patients (Shaldon, 1966; Eschbach, Funk, Adamson, Kuhn, Scribner & Finch, 1967; Eschbach, Cook & Finch, 1970). The use of a whole-body monitor permitted measurement in a single study of the oral absorption of a tracer dose of 59 Fe, its incorporation into erythrocytes and the subsequent long-term rate of loss of 59 Fe from the body. We have previously reported the results of an investigation of iron metabolism by using this method in patients with chronic renal failure (Boddy et al. , 1970). We now report the results of a similar investigation of iron metabolism in patients with chronic renal failure undergoing regular haemodialysis therapy.