Numerous epidemiological studies have pointed out a higher frequency of temporomandibular disorder (TMD) in women than in men, which indicates the involvement of a sex hormone, such as estrogen, in the pathogenesis of TMD. Although estrogen is known to play pivotal roles in osteoarthrosis or rheumatoid arthritis in systemic joints, there have been few reports about the role of estrogen in the temporomandibular joint (TMJ). The effect of estrogen is generally mediated by the estrogen receptors (ERs) ERα (the predominant type) and ERβ. In this study we examined the expression of ERα protein and mRNA in the TMJ of adult male rats by immunocytochemistry and in situ hybridization histochemistry. Intense ERα immunoreactivity was localized in the synovial lining cells, stromal cells in the articular disc, and chondrocytes in the TMJ. These ERα-immunopositive synovial lining cells are characteristic of cytoplasmic processes identified with confocal and immunoelectron microscopy, which indicates that they are synovial type B cells. In situ hybridization histochemistry confirmed intense signals for ERα in the synovial lining cells and the sublining fibroblasts at mRNA levels. The nuclei of chondrocytes showed an intense immunoreaction for ERα in the maturative and hypertrophic layers of the articular cartilage. In addition to the nuclear localization of ERα, a weak immunoreaction appeared in the cytoplasm of some ERα-positive cells. These findings support the hypothesis that TMJ tissue—at least in the male rat—has the potential to be an estrogen target tissue. Anat Rec Part A 274A:934–941, 2003. © 2003 Wiley-Liss, Inc.