Ultraviolet (UV) irradiation from the sun reduces production of type I procollagen (COLI), the major structural protein in human skin. This reduction is a key feature of the pathophysiology of premature skin aging (photoaging). Photoaging is the most common form of skin damage and is associated with skin carcinoma. TGF-beta/Smad pathway is the major regulator of type I procollagen synthesis in human skin. We have previously reported that UV irradiation impairs transforming growth factor-beta (TGF-beta)/Smad signaling in mink lung epithelial cells. We have investigated the mechanism of UV irradiation impairment of the TGF-beta/Smad pathway and the impact of this impairment on type I procollagen production in human skin fibroblasts, the major collagen-producing cells in skin. We report here that UV irradiation impairs TGF-beta/Smad pathway in human skin by down-regulation of TGF-beta type II receptor (TbetaRII). This loss of TbetaRII occurs within 8 hours after UV irradiation and precedes down-regulation of type I procollagen expression in human skin in vivo. In human skin fibroblasts, UV-induced TbetaRII down-regulation is mediated by transcriptional repression and results in 90% reduction of specific, cell-surface binding of TGF-beta. This loss of TbetaRII prevents downstream activation of Smad2/3 by TGF-beta, thereby reducing expression of type I procollagen. Preventing loss of TbetaRII by overexpression protects against UV inhibition of type I procollagen gene expression in human skin fibroblasts. UV-induced down-regulation of TbetaRII, with attendant reduction of type I procollagen production, is a critical molecular mechanism in the pathophysiology of photoaging.