1. Cardiac index, oxygen uptake and haemoglobin concentration have been measured in eight patients with end-stage renal failure, and the values compared with those in eight healthy control subjects. Assuming the arterial blood to be 97% saturated, the oxygen saturation of mixed venous blood has been calculated. 2. Cardiac index was lower in the patients (2.45 ± 0.42 litres min −1 m −2 ) compared with the control subjects (3.74 ± 0.17 litres min −1 m −2 ). Oxygen uptake was reduced from 147 ± 16 ml STP min −1 m −2 in the control subjects to 112 ± 9 ml STP min −1 m −2 in the patients. 3. Oxygen saturation of mixed venous blood was 53 ± 8% in the patients compared with 79 ± 2% in the control subjects, suggesting some degree of tissue hypoxia. If oxygen uptake were not reduced in renal failure, then the mixed venous blood oxygen saturation would be as low as 40%. 4. The oxygen saturation of blood sampled from the superior vena cava of patients with renal failure was 57 ± 6%. 5. The data on oxygen uptake and venous oxygen saturation suggest that in the patients with end-stage renal failure, the low values of cardiac index, previously measured by impedance cardiography, are accurate.