Photodynamically induced vitreous liquefaction in rabbit eye was investigated. Photosensitizer, riboflavine phosphate, was injected into the vitreous cavity of the rabbit before white-light irradiation. After the irradiation (0, 1, 3, 6 hr) the rabbit vitreous body was separated into gel and liquid portions. The liquid vitreous body was weighed, and the vitreous liquefaction percentage was calculated. One hour irradiation caused 38% of liquefaction of the eye; 3 hr, 50% liquefaction; 6 hr, 59% liquefaction. Although irradiated control eye (without photosensitizer) and the dark adapted control eye (non-irradiated) showed 10-15% liquefaction throughout the experimental period, the liquefaction percentage of the experimental vitreous bodies was significantly larger than that of the control vitreous. Inhibition experiment showed that the radical scavengers (Cu, Zn-superoxide dismutase, catalase, and mannitol) could suppress the photodynamically induced vitreous liquefaction. Results indicated that free radicals, including hydroxyl radical and superoxide anion, which are generated by photosensitizer and visible light irradiation, may contribute to the age-related vitreous liquefaction of humans.