The morphology of the vitreoretinal border region, also termed the inner limiting membrane, was examined in spontaneously diabetic rats (BB rats), in non-diabetes-prone rats (WB rats) and in Buffalo rats (BUF rats) by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This was performed in order to visualize a possible increase in thickness of the lamina densa or in the whole vitreoretinal border region complex with duration of diabetes. The median thickness of the lamina densa in the three groups varied between 34 and 68 nm. In BB rats the thickness decreased with age and duration of diabetes. In WB rats the lamina densa thickened up to the 9th month and then decreased to the level of the young rats. In BUF rats the lamina densa decreased in thickness with age. The median thickness of the whole vitreoretinal border region varied between: BB rats: 84 and 126 nm (SEM) and 68 and 119 nm (TEM); WB rats: 42 and 84 nm (SEM) and 51 and 68 nm (TEM); BUF rats: 42 nm (SEM) and 34 and 68 nm (TEM). In BB rats the whole vitreoretinal border region appeared in SEM to thicken with duration of diabetes > or = 27 weeks, whereas in TEM no specific pattern could be statistically demonstrated. In WB rats the whole vitreoretinal border region was thinner in 3-month-old animals than in 9-month-old animals in (SEM) and TEM. In BUF rats there was no difference in thickness between young and older animals. Light microscopy of older diabetic animals showed neovascularization of the iris stroma.