Increased capillary permeability to small and large molecules has been demonstrated in several organs of human diabetics and in animal models of the disease. To assess the effects of diabetic microangiopathy on peritoneal clearances, peritoneal dialysis was performed in normal rats, rats with gentamicin-induced acute renal failure, and rats with mild and severe alloxan-induced diabetes mellitus. Peritoneal clearances of urea, inulin, and albumin were significantly increased in the severely diabetic rats compared with other groups. Microscopic examination revealed abnormally small adipose cells in the peritoneums of the severely diabetic rats, associated with a generally malnourished appearance. Vascular abnormalities attributable to diabetes mellitus, such as capillary basement membrane layering and neovascularization, were demonstrated in the peritoneums+ of both mildly and severely diabetic rats, though not to a greater extent in either group. We conclude that, as in many other parts of the body, peritoneal permeability to small, middle and large molecules is increased in early, severe, untreated diabetes mellitus.