The paracrine linkage of endothelins (ET) between keratinocytes and melanocytes suggested that ETs are intrinsic mediators for human melanocytes in UVB-induced pigmentation. In this study, the role of ET-1 in the epidermal hyperpigmentation was investigated in vivo and in vitro. The addition of 10 nM ET-1 induced a H-7 (10 microM) suppressible-increase in tyrosinase activity in cultured human melanocytes and was accompanied by elevated levels of tyrosinase and tyrosinase-related protein-1 mRNA expression as shown by Northern blotting. Analysis of signaling mechanisms leading to tyrosinase activation demonstrated the involvements of quick translocation of PKC, the H-7 (10 microM) suppressible-phosphorylation of the threonine residue of several proteins, and highly elevated level of cyclic AMP (4-fold over control). Reverse transcription polymerase chain reaction (RT-PCR) of RNA isolated from the epidermis of human skin exposed to UVB revealed that UVB irradiation with a dose of 2 MED caused a significant increase in the expressions of ET-1, IL-1 alpha, and tyrosinase mRNA signals 5 days after irradiation. The involvement of ET-1 in UVB-pigmentation was also corroborated by the experiments that the extracts of M. Chamomilla, which can act as an antagonist for ET-receptor binding-mediated signaling but has no inhibitory effect on tyrosinase activity in culture, had a significant inhibitory effect on UVB-induced pigmentation in vivo when daily applied immediately after UVB exposure to human skin. These findings suggest that ET-1 is an important mediator in the epidermis for UVB-induced pigmentation in vivo.