Genetically diabetic mice (C57 BLKsJ db/db) aged 5-6 weeks were given a diet containing 20% by weight of non-nutritive bulk and compared with age matched control diabetic mice on a normal diet (fibre content 4.5%) and non-diabetic mice. The duration of study was 12 weeks. No adverse effects were observed in animals given the high fibre diet. Total food consumption was greater in mice receiving the fibre enriched diet, but their absolute caloric intake was 6% less than control diabetic mice. Both groups exhibited similar rates of growth and development. Water intake in the experimental diabetic mice was reduced and similar to that of normal non-diabetic mice. Fasting blood glucose was significantly decreased in the experimental diabetic mice at 12 weeks. Renal pathological lesions in the control diabetic mice showed glomerular mesangial expansion and deposition of immunoglobulins within the mesangium. The experimental diabetic animals exhibited significantly less renal pathology, including light and immunofluorescent lesions. It is concluded that addition of non-absorbable fibre to the diet of genetically diabetic mice improves glycaemic control and retards the development of diabetic nephropathy.