Purpose: To clarify the mechanism underlying the development of cataract in the rat lens after intraperitoneal administration of iodoacetic acid (IAA). Methods: (1) The 2% IAA dissolved in saline solution was injected at a dose of 40 mg/kg body weight into the rat peritoneal cavity. The retina and lens were intermittently extirpated and were examined by light and electron microscopy. (2) Two kinds of tracer, Evans blue (EB) and horseradish peroxidase (HRP), were injected into the tail veins and anterior chamber, and were observed with dissecting and electron microscopes. Results: (1) Four weeks after administration, a part of the lens epithelium at the lateral side of the lens was degenerated, and the lens nucleus developed faint turbidity after 8 weeks. After 16 weeks, the nuclear turbidity could not be observed because mild cortical opacity was developing. The epithelial degeneration recovered from around 12 weeks, and instead of spherical nuclei, elliptical nuclei appeared. (2) The EB dye injected into the tail vein significantly stained the ciliary body, where the anterior and posterior ciliary arteries anastomosed. EB injected from the lateral side of the lens was seen to move towards the lens nucleus. Electron microscopically, the epithelial degeneration of the ciliary body was observed. The incorporated HRP substance was found in the cytoplasm of the nonpigmented cells of the ciliary epithelium at an early stage after IAA administration. Conclusion: IAA injected intravenously first developed epithelial degeneration at the lateral side of the lens. This change induced swelling of the lens fibers in the lens nucleus. Recovered epithelial cells had a transformed nucleus, and in turn the cortical cataract was induced by a differentiation disorder of the lens fibers. These results indicate that the breakdown of the blood-aqueous barrier in the nonpigmented epithelium of the ciliary body is a trigger to cause the cataract. The IAA-induced cataract may be useful as an animal model of human age-related cataract.