Fifty male albino rabbits were studied. Ten animals served as controls. Forty animals were prepared to receive photorefractive keratectomy (PRK), including anesthesia, scraping of the corneal epithelial cells, etc. Twenty of these animals then received PRK with energy delivered by excimer laser (pulse rate = 20 Hz, fluence 250 mJ/cm2; number of pulses = 6032; cumulative UV dose = 1508J/cm2). The other 20 animals were exposed to the same operating microscope light as the PRK-treated animals, but they did not receive PRK. All three groups were divided into halves: the first half was immediately analyzed at 0 time, and the second half was observed 1 year later. Samples of aqueous humor and lens were analyzed. The levels of reduced and oxidized glutathione, hydrogen peroxide, ascorbic acid, and malondialdehyde were determined. Excimer laser-induced ultrastructural modifications of the lens, verified through scanning electron microscopy, were studied at the same intervals. Immediately after PRK, the biochemical parameters studied, both in aqueous humor and in lens of treated animals, showed significant differences. One year later, the observed biochemical variations in lens were still present, whereas aqueous humor values did not significantly differ from control values. Ultrastructural abnormalities of the lens appeared only 1 year after PRK In the animals that received only the preparation for PRK the biochemical and ultrastructural differences did not significantly vary as compared to the data obtained from control animals. These findings suggest that the biochemical and ultrastructural lens alterations induced by PRK may represent events relevant to cataractogenesis in the rabbit.