Overexpression of extracellular matrix (ECM) components is closely associated with the development of vascular basement membrane (BM) thickening, a histological hallmark of diabetic microangiopathy. To determine whether BM thickening of retinal capillaries could be prevented by down regulating synthesis of fibronectin, an ECM component, we used antisense oligos targeted against translation initiation site of the fibronectin transcript in galactose-fed rat, an animal model of diabetic retinopathy. After 2 months of galactose-feeding, intravitreal administration of 3 mol/l antisense fibronectin oligos was initiated at monthly intervals for 3 months. The antisense strategy significantly reduced fibronectin mRNA and protein level in the retinas of treated eyes compared with untreated eyes of galactose-fed rats (130 ؎ 16 vs. 179 ؎ 18% of control, P < 0.01, and 144 ؎ 28 vs. 204 ؎ 22% of control, respectively, r ‫؍‬ 0.9) and resulted in partial reduction of retinal capillary BM width (123 ؎ 16 vs. 201 ؎ 12 nm, P < 0.03). In eyes treated with antisense fibronectin oligos, ϳ35% reduction in both pericyte loss and acellular retinal capillaries was observed (P < 0.04 and P < 0.03, respectively). Glycohemoglobin level was consistently elevated in the treated (6.9 ؎ 0.6%) and untreated (6.5 ؎ 0.7%) galactose-fed rats compared with control rats (4.5 ؎ 0.8%). Overall, these results indicate that downregulation of fibronectin synthesis reduces BM thickening in retinal capillaries with beneficial effect to retinal lesions. The antisense fibronectin oligos may provide a useful approach for reducing vascular lesions in diabetic retinopathy. The thickened vascular BM may be a potential therapeutic target for preventing retinal lesions in diabetic retinopathy. Diabetes 52: 1229 -1234, 2003