Background: Albuminuria is a cardinal sign of disruption to the glomerular filtration barrier (GFB), and it is the earliest marker of renal involvement in diabetes and the hyperinsulinaemic metabolic syndrome, which are conditions secondary to a failure of the production, or cellular action, of insulin. Methods/findings: We have studied the direct effect of insulin on the healthy glomerular filtration barrier using in vivo and in vitro techniques. In vivo- Wistar rats were treated with high physiological doses of insulin (1000pM) and their blood glucose levels maintained with a glucose infusion. These were compared to control animals given a saline infusion. Within 30 minutes insulin treated rats increased their urinary albumin excretion by 60% (p<0.01). Animals were sacrificed and their kidneys perfusion fixed. Insulin treated animals showed widening of their podocyte foot processes with less filtration slits per standardised unit length of filtration area (p<0.05). There was also underlying swelling and vacuolation of endothelial cells. In vitro- Conditionally immortalised human glomerular podocyte and endothelial cell lines were examined. Human podocytes reorganised their actin cytoskeleton from stress fibre to cortical patterning within 15 minutes (phalloidin staining), with associated retraction of the fine processes of the cells (atomic force microscopy and real time imaging). Within 5 minutes, exclusively in podocytes, insulin switched on the small GTPase RhoA and switched off Rac1 and CDC42 (the molecular switches for actin reorganisation). Insulin also directly caused a functional loss of resistance across podocyte monolayers, but not endothelial cells (Electrical cell surface impedance sensing). Conclusions: Insulin has a previously unsuspected direct remodelling effect on the glomerular filtration barrier of the healthy kidney. As the earliest manifestation of renal involvement in diabetes and the metabolic syndrome is albuminuria, loss of mechanism could be of great importance and an early target for therapy.