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Oestrogens are responsible for the sexual dimorphism in adult mice lung tissue, and this difference is most notable at sexual maturity. Oestrogen receptor-alpha (ERα) and the oestrogen receptor-beta (ERβ) are the two receptors that mediate oestrogen action, but adult mice lung tissue only expresses ERβ, and it is probably through this receptor that oestrogens exert their action. The goal of our study was to detect the expression of ERα and ERβ in mouse foetal lung tissues and identify possible gender differences. The foetal lung tissue was collected between developmental days E15–E19, processed for histology and the expression of oestrogen receptors was detected by immunohistochemistry. Over the 5 days of lung development that were evaluated ERα was not expressed in the foetal lung tissues of neither male nor female mice. In contrast, ERβ was detected in both sexes, although the immunoreactivity differed for each developmental day whilst the staining intensity observed for ERβ also indicated differences between male and female lung tissues. The results demonstrate the existence of a gender difference in the foetal expression of ERβ in lungs of mice.