Abstract: Objective To compare the skin phototoxicity damage of SD rats induced by ultraviolet radiation at different radiation time. Methods The skin phototoxicity damage rat models were established by ultraviolet radiation of UVA (4.5mJ·cm^-2·s^-1)+UVB(0.036 mJ·cm^-2·s^-1). The models were irradiated twice a week for 4 weeks, and the cumulative radiation time of model 1, model 2 and model 3 were 342 min, 440 min and 520 min, respectively. At the end of the experiment, the skins were histopathologically observed, as well as the expression of melanoma specific marker (HMB45), Vimentin and endothelial cell marker (CD34) in skins were examined by immunohistochemistry. Results Compared to normal control, there showed partial hyperkeratosis of epidermis, uneven thickness of epidermis, deepening of wrinkle, enlargement of hair follicle, and hyperplasia of sebaceous gland in model 1, model 2 and model 3. In addition, with the increase of radiation time, the epidermal lesions were more serious, the skin thickness and sebaceous gland cross-sectional area were larger, and the skin injury scores were higher. Compared with normal control, the expressions of Vimentin of skin flbroblast and CD34 of Dermal capillaries increased tremendously in model 1, model 2 and model 3, and the increasement were more obviously in model 2 and model 3. At the same time, the expressions of HMB45 of skin increased distinctly in model 2 and model 3. Conclusion With the extension of ultraviolet radiation time, the degree of skin damage is gradually increased. At the same time, this study may provide an appropriate animal model for evaluating the efficacy of sunscreen.

Key words: UVA, UVB, SD rats, Skin Phototoxicity