Ultrastructural changes in the dorsal skin were examined in Wistar-derived hypotrichotic WBN/ILA-Ht rats exposed to subchronic UVB-irradiation (10 kJ/m2 per rat per day for up to 3 months). Epidermal hyperplasia developed at I month of UVB-irradiation and progressed thereafter, resulting in epidermal thickening and formation of epidermal ingrowths projecting into the dermis. In some portions of the epidermal ingrowths at 2 and 3 months, keratinocytes were somewhat pleomorphic. In addition, some of the keratinocytes showing cytoplasmic projections migrated into the dermis. The basement membrane and hemidesmosomes at the epidermal-dermal junction became to disappear along with the development of edema spreading from the upper dermis to the epidermis. However, Langerhans cells were still detected in the hyperplastic epidermis even at 3 months. In the dermis, in addition to edema, fibroblast proliferation and mast cell infiltration progressed with time, and degranulation of mast cells was obvious at 2 and 3 months. Only a few basophils as well as eosinophils were also found. In the upper dermis, especially beneath the epidermis, decrease in diameter and disintegration of collagen fibrils were observed. Ultrastructural characteristics of the dorsal skin responses to subchronic UVB-irradiation were clarified in the present study.