1. The skeletal muscle capillary permeability to 131I− and [51Cr]EDTA was studied in the exercising human forearm of seventeen non-diabetics and seventeen long-term juvenile diabetics with clinical signs of diabetic angiopathy. 2. The permeability data was obtained using the indicator diffusion technique, which implies intra-arterial injection and collection of multiple venous samples during the first passage of the tracers through the organ. The capillary permeability was expressed as the capillary diffusion capacity, CDC, i.e. the maximum unidirectional flux of tracer over the capillary membrane of 100 g of muscle per unit concentration difference over the capillary. 3. The average values for both CDCI and CDCCr-EDTA were found to be significantly increased (P < 0·001) in long-term diabetics as opposed to non-diabetics: CDC1 = 18·7 (SD, 3·2), 13·2 (SD, 1·0); CDCCr-EDTA = 6·74 (SD, 1·09), 3·73 (SD, 0·30)mol 100g−1 min−1 per mol ml−1 respectively. 4. The findings indicate an increased skeletal muscle capillary permeability in long-term diabetics which most probably is due to an increased permeability per unit capillary surface area.