Dorsal skin reactions to irradiation with two different types of ultraviolet (UV) light sources (sunlight and artificial UV light) were investigated in hairless descendants of Mexican hairless dogs. The total energy dose of solar UV exposure and artificial UV irradiation was 90 kJ/m2 and 40 kJ/m2, respectively. Histological examinations were done up to 7 days after UV irradiation (7 DAI). At 1 DAI, the solar UV-exposed skin did not show marked changes, while artificial UV irradiated skin exhibited a visible erythematous reaction and prominent histological alterations such as epidermal thickening, appearance of sunburn cells and deformation elastic fibers. At 4 DAI of solar UV-exposure, the skin color became moderately dark and noticeable pigmentation developed in the epidermis. In contrast, at 4 DAI of artificial UV irradiation, there still remained moderate degeneration in the epidermis and dermis, and delayed tanning was weak. At 7 DAI of solar exposure, delayed suntan reactions became more prominent. Histologically, there were heavy pigmentation of melanin granules mainly in the stratum basale. On the other hand, artificial UV irradiated skin showed less pigmentation. Thus, solar exposure provoked remarkable pigmentation while artificial UV irradiation brought about severe sunburn reactions in the dorsal skin of hairless dogs.