Repeated ultraviolet light (UV) irradiation on the skin of rats caused ulceration, scarring and epithelial tumors; 38% of the animals had ear tumors and 5% skin tumors. Guinea pigs presented minimal evidence of neoplastic transformation; only two tumors were seen while hamsters displayed localized epidermal hyperplasia, and 35% of the animals had papillomas and keratoacanthomas of the dorsal skin. In Swiss mice UV irradiation caused ulceration, necrosis and hyperplasia; moreover, fibromas and fibrosarcomas occurred in 17 of 20 tumor-bearing mice. Epithelial tumors originated from the proliferation of benign epidermal cells, through dysplastic changes in the surface epithelium, or from the borders of skin ulcers (in rats). These tumors were local and circumscribed in the hamsters, while covering the entire ear tip in rats. Dermal tumors, consisting of fibroblastic 'light' and 'dark' cells, occurred in mice previously showing extensive ulceration and scarring.