Changes in glomerular permeability, manifesting as albuminuria, occur early in diabetic nephropathy. The endothelial glycocalyx is an important regulator of glomerular permeability, and is disrupted in diabetic patients with albuminuria (Nieuwdorp et al (2006). Which component of the endothelial glycocalyx contributes to altered glomerular permeability in diabetes is not known. We have examined the roles of hyaluronan and chondroitin sulphate in regulating glomerular water permeability in healthy and diabetic animals. Male Sprague Dawley rats received sodium citrate (sham) or streptozotocin (45 mg/kg) i.v. to induce diabetes. On day 7, rats were anaesthetised (sodium pentobarbital) and kidneys were perfused via the abdominal aorta with Ringer-Albumin solution. Right renal perfusate also contained Octadecyl-Rhodamine-B Chloride (R18: cell membrane label) and either Chondroitinase ABC (7.25U/kg) or Hyaluronidase (15000U/kg). Successful selective glomerular perfusion was confirmed with confocal microscopy. Following exsanguination and nephrectomy, glomeruli were sieved from the cortex of each kidney, aspirated onto a micropipette and equilibrated in 1% BSA perifusate. Perifusate switch to 8% BSA caused glomeruli to shrink. The initial rate of change in glomerular volume was recorded, calculated off-line, and used to determine volume-corrected glomerular water permeability (LpA/Vi: min-1 mmHg-1), assuming σ=1. One-way ANOVA (Bonferroni correction) was used to compare mean±sem(n) LpA/Vi between groups. Glomerular LpA/Vi was increased in diabetic rats (sham: 0.85±0.08(58); streptozotocin: 1.23±0.13(39); p<0.0001). Neither chondroitinase nor hyaluronidase significantly altered LpA/Vi in sham- treated animals (chondroitinase: 0.844±0.08(28); hyaluronidase: 0.678±0.06(30); both p>0.05 vs vehicle-sham). In diabetic animals, only hyaluronidase modified LpA/Vi (chondroitinase: 1.22±0.27(19); hyaluronidase: 0.674±0.10(19)*; *p>0.05 vs vehicle-streptozotocin). Glomerular water permeability (LpA/Vi) is increased in diabetic animals. Hyaluronan appears to be an important contributor to this diabetes-related change in glomerular water permeability.