The first results of a long-term study on rats rendered chronically diabetic with streptozotocin are presented. Besides well-known ocular changes (formation of cataracts, refractive changes, neovascularisation of iris, loss of retinal capillary endothelial and mural cells; variations in capillary diameter due to fusiform capillary microaneurysms), the formation of typical saccular microaneurysms, qualitative and quantitative changes of the capillary basement membrane ultrastructure have been observed. The basement membranes of diabetic animals showed focal thickenings containing basement membranelike and/or fibrillar material with the periodicity of collagen, but an increase of mean basement membrane width was not found compared with the controls.