Estrogens are considered essential to the mechanism for closure of epiphyses in both males and females. The mechanism for this, however, is still unclear. It is likely that estrogen acts directly on growth plate chondrocytes, but the localization of the cells expressing the estrogen receptor (ER) has yet to be ascertained. Moreover, in rodents, growth plates remain open well into adult life. Whether the distribution of estrogen target cells in rodent epiphyses differs from that in other species, is also unclear. We therefore compared localization of estrogen target cells (denoted by ER-alpha protein expression) in species in which growth plates fuse, with that in rodents. Thus, we have investigated ER-alpha protein expression in femoral growth plates from male and female rabbits, just at sexual maturity (6 months), when growth plate fusion was just commencing, and in rats of equivalent developmental stage (9 weeks). ER-alpha was detected in undecalcified cryosections by immunofluorescence with 1D5 monoclonal antibody, raised to human ER-alpha; uterine sections were positive controls. ER-alpha-positive cells were localized to the proliferative/early hypertrophic zone of male and female rabbits. By contrast, cells in the similar region of the mature rat growth plate were ER-alpha-negative in both genders, although receptor could be readily detected in uteri of mature female rats. In growth plates of immature male and female rats (6 weeks), however, ER-alpha was clearly expressed by cells of the proliferative/early hypertrophic zone, but was barely detectable in uteri from immature females. Our findings support the view that estrogen may act directly on the growth plate and, in species in which there is epiphyseal fusion, may thus have a role in this process. If ER-alpha expression is lost at sexual maturity, as in rodents, growth plates may remain open into adulthood. Our results also show the changes in ER-alpha expression in growth plates of maturing rats may be opposite to that in the uterus and raise the possibility that receptor expression may be controlled differently in reproductive and skeletal tissues.