Morphological and numerical changes in the epidermal melanocytes of black C57BL mice after phototoxic drug administration followed by ultraviolet A irradiation were studied to compare the effects of photochemotherapy on the epidermal melanocytes using 8-methoxypsoralen and trimethylpsoralen. One hour after intraperitoneal injection of the phototoxic drugs, 1.5 mg/kg in the small dose group and 6.0 mg/kg in the large dose group, the mice were exposed to UVA irradiation. This procedure was performed twice a week for 8 weeks at the small dose group and for 5 weeks in the large dose group. Skin biopsies were taken before irradiation in both groups, and follow up biopsies were done at each week. The number and size of the melanocytes were observed in a split-DOPA preparation. In the drug treated groups, there was an increase in the size of the perikaryon, and the number, length, width, and arborization of dendrites. Such changes were more clearly seen in the group treated with trimethylpsoralen compared with the 8-methoxypsoralen treated group. Therefore, trimethylpsoralen is more effective than 8-methoxypsoralen in the increase of the perikaryon size, and the number, length, width, and arborization of dendrites of melanocytes in the intraperitoneal injection.