In order to examine the permeability of microvessels in diabetic children, the glomerular filtration rate, urinary excretion rates of albumin and beta 2-microglobulin, intravascular mass of albumin, and transcapillary escape rate of albumin were studied in 26 diabetic children without clinical signs of microangiopathy (age: 7-14 years; duration of disease: 3-14 years). Similar measurements were made in 28 healthy school children (age: 8-14 years). Mean glomerular filtration rate in the diabetic children was higher than in the normal children (138 versus 109 ml/min per 1.73 m2, p less than 0.01). Urinary excretion rates of albumin and beta 2-microglobulin did not differ in diabetics. Mean intravascular albumin mass in the diabetic girls (1.64 g/kg body weight) was lower (p less than 0.01) than in the diabetic boys (1.89 g/kg body weight) and also lower (p less than 0.02) than in the normal girls (1.94 g/kg body weight). Mean transcapillary escape rate of albumin in the twenty diabetics with duration of diabetes less than 10 years (7.14%/h) was lower (p less than 0.01) than that in normal children (8.90%/h); the escape rate showed a positive correlation with duration of diabetes (r=0.47; p less than 0.02). Thus glomerular filtration rate in diabetic children is elevated to the same extent as in adult short-term juvenile diabetics while the permeability of the glomerular membrane to macromolecules is normal. Interpretation of the results on intravascular albumin mass and transcapillary escape rate of albumin requires further investigation.