The effect of intravenous insulin (7-8 U as a bolus injection) on renal haemodynamics and urinary excretion of albumin and beta-2-microglobulin was examined in five recent onset juvenile diabetics. Blood glucose concentration was maintained after insulin at unchanged or slightly increased levels by continuous intravenous glucose infusion (50 g/100 ml, 1.2 ml/min). Mean arterial blood pressure increased slightly but significantly from 94 +/- 8 mmHg to 99 +/- 10 (mean +/- SD) after insulin. The rise in heart rate (16 versus 29 beats/min) and in plasma noradrenaline (from 0.16 to 0.32 ng/ml versus 0.20 to 0.49 ng/ml) was significantly greater in the tilted position after insulin. There was no decrease in glomerular filtration rate or renal plasma flow after insulin, in contrast to the findings after intravenous injection of insulin without maintenance of plasma glucose. Urinary albumin excretion was approximately doubled after insulin, from 6.8 to 12.5 microgram/min. Beta-2-microglobulin excretion decreased but this difference was not significant. -- It is concluded that the rise in heart rate and plasma noradrenaline, and the increase in urinary albumin excretion, after insulin, are unrelated to changes in blood glucose concentration. It is suggested that increased albumin excretion after insulin is due to a direct effect of insulin on glomerular endothelial or epithelial cells.