The internal limiting membrane (ILM) and cortical vitreous of the rabbit and primate were studied with transmission electron microscopy following staining with the cationic dye, Alcian blue GX. An unusual feature of the cortical vitreal collagen fibril was its displacement from the ILM: it did not insert into the lamina densa. The separation between vitreal collagen and the ILM was especially noticeable in the rabbit eye, which possessed an extremely strong vitreal retinal attachment in the posterior fundus. The lack of fibril insertion was observed in rabbit tissue that had been fixed by quick freezing on a helium-cooled copper block. The similarity in the appearance of tissue fixed by glutaraldehyde, glutaraldehyde supplemented with Alcian blue, or by quick freezing suggested that the lack of collagen fibril insertion into the ILM was an accurate representation of the relationship between collagen and the ILM. It was found that these two animal species had radically different ILM's; the rabbit ILM was a thin basement membrane throughout all areas of the posterior fundus, whereas the ILM of the cynomolgus monkey was a thick basement membrane in the peripapillary region and a thin basement membrane in the region of the fovea centralis. The topographic variations in the primate ILM thickness and appearance followed the pattern observed in human eyes. Like man, the thickening of the cynomolgus ILM in the posterior fundus was age related. The similarity between the cynomolgus and human ILM suggests that this animal would be more suitable than the rabbit for studying age-related changes or alterations in the strength of vitreal attachment following trauma.