1. Capillary diffusion capacity (CDC) for 131I in gastrocnemius muscle was investigated by the tissue clearance method of Lassen (1967) in thirty-three patients with various degrees of impairment of renal function.
2. The mean CDC in uraemic patients [16·9 mol/(mol/ml)/100 g/min] was significantly higher than in the control group [7·1 mol/(mol/ml)/100 g/min]. A comparison of patients with renal failure due to chronic glomerulonephritis with those in whom it was due to chronic pyelonephritis showed no significant difference.
3. The most likely interpretation of these observations is that capillary permeability is increased in uraemia.