Abnormalities of collagen synthesis [1] and increased oxidative stress [2] have been implicated in the pathogenesis of diabetic microangiopathy. Since ascorbic acid (AA; vitamin C) is an essential cofactor for prolyl hydroxylase, a key enzyme in the biosynthesis of collagen and is also an important antioxidant [3], AA metabolism may link these processes.Decreased plasma AA concentrations and increased turnover of AA to the oxidised metabolite dehydroascorbic acid (DHAA) have been reported in diabetic subjects [4±10], particularly in patients with poor glycaemic control [11] and/or microangiopathy [12,13]. Although (i) leucocyte AA concentration is reduced following intravenous glucose [14], (ii) in vitro cell culture studies [15±18] demonstrate competitive cellular uptake of glucose and AA and (iii) in Diabetologia (1998) Summary Previous studies demonstrating reduced plasma concentrations of ascorbic acid (AA) in diabetes and interactions between this vitamin and biochemical mechanisms such as synthesis of structural proteins, oxidative stress, polyol pathway and nonenzymatic glycation of proteins suggest that disturbed AA metabolism may be important in the pathogenesis of diabetic microangiopathy. However, limited information is available on the concentration of AA in tissues which develop diabetic complications. This study demonstrates reduced renal but not sciatic nerve or plasma AA concentration in two animal models of insulin-dependent diabetes mellitus, namely the STZ-diabetic rat and the spontaneously diabetic BB rat. Decreased lens AA concentration was also observed in STZ-diabetic rats. Improvement of glycaemic control by insulin treatment (albeit insufficient to achieve normoglycaemia) partially corrected lens and renal AA concentration in STZ-diabetic rats. AA treatment increased kidney and lens AA concentrations of STZ-diabetic and non-diabetic rats and corrected the abnormalities observed for untreated diabetic rats. Sciatic nerve AA concentration was not increased by AA treatment in any group. Tissue ratios of dehydroascorbic acid (DHAA)/AA, one index of oxidative stress, were not different between the diabetic and non-diabetic groups and were unaltered by AA supplementation. AA treatment of STZ-diabetic rats had no effect on elevated tissue concentrations of glucose, sorbitol and fructose or reduced myo-inositol concentration. The effect of reduced tissue AA levels in diabetes on either collagen synthesis or ability to combat increased free radical production is not known. However, correction of abnormal kidney and lens AA concentrations in experimental diabetes by AA supplementation suggests that if AA does have a role in the development or progression of the renal and ocular complications of diabetes, this treatment could be beneficial. [Diabetologia (1998) 41: 516±523]