The effects of oestrogens and androgens, alone or in combination, on several epididymal parameters have been studied in 15-day-old rats after neonatal treatment. Oestrogens induced several responses, such as increased growth of the fibromuscular stroma and eosinophil leucocyte accumulation, whereas the proliferative activity of the epithelium was decreased significantly. Otherwise, the density of intra-epithelial leucocytes was not modified. Different oestrogen-induced responses, such as the increase in volume of the fibromuscular stroma and eosinophil leucocyte accumulation were inhibited by treatment with testosterone, whereas dihydrotestosterone had no appreciable effect. This study raises the possibility that eosinophils are mediators of some of the oestrogenic responses in the early postnatal rat epididymis.