The effects of high glucose concentrations on the selective permeability of a layer of cultured bovine vascular endothelial cells (ECs) were investigated. ECs were derived from the intima of the thoracic aorta and permeability to nonglycated albumin, glycated albumin, and fluorescein dextrans (FDs) of molecular mass to albumin was measured. ECs were cultured on a filter coated with type I collagen and preincubated in the presence of various glucose concentrations for 96 h. Human serum albumin was glycated by incubation with glucose in vitro. Nonglycated and glycated albumin were separated by affinity column chromatography. The permeation rates of nonglycated and glycated albumin as well as those of neutral and anionic FD through the EC layer were increased by preincubation of cells with high glucose concentrations (22.2 and 44.4 mmol/l). The permeation rate of glycated albumin was significantly less than that of nonglycated albumin at all glucose concentrations tested, whereas the permeation rate of anionic FD was significantly lower than that of neutral FD only at a physiological glucose concentration (5.6 mmol/l). The aldose reductase inhibitor ponalrestat partially inhibited the high glucose-induced increase in trans-endothelial permeation of albumin (both nonglycated and glycated), but had no effect on the increased permeation of FD. These results indicate that high glucose concentrations enhance trans-endothelial permeability to albumin and FD and may disturb the barrier function of vascular ECs. Furthermore, metabolism of glucose via the polyol pathway may contribute to abnormalities in trans-endothelial permeability.