The retinal vessels of experimental renal hypertensive rats (RHRS), spontaneously hypertensive rats (SHRS), and normotensive Wistar-Kyoto rats (WKYS) were studied using bis-benzimide vital staining. In the bis-benzimide stained flat retinal preparations (B-FRP) of SHRS, it was only observed that the endothelial nuclei were elongated and their arrangement was irregular. In B-FRPS of RHRS, the arrangement was more irregular and the nuclei disappeared at the point where the caliber irregularity was marked. The nuclei of smooth muscle cells, which were arranged circumferentially, and the perivascular tissues were clearly identified. This finding suggests that hyperpermeability or disruption of the blood-retinal barrier did not occur in the mild hypertension but in the severe hypertension. The retinal vessels of RHRS in the regressive stage also showed seved the same kinds of findings in electron microscopy.