A number of recent studies suggest the effects of several growth factors as well as genetic factors on the development and differentiation of the testes and excretory ducts. One of the most important growth factors affecting the development and differentiation of the above-mentioned organs may be epidermal growth factor (EGF). Therefore, the aim of the present study was to investigate the effects of EGF on the development of the testes and excretory ducts of male rats by determining the immunolocalization of EGF receptors in these organs during foetal and postnatal periods. To do this, testicular and excretory duct tissue samples from prenatals of 13, 16 and 18 days, neonates, 10-day-olds and adults were obtained and processed for light microscopy for immunohistochemical examination. APAAP immunohistochemical staining was applied to 5-7mm thick paraffin sections. While there was a strong immunoreactivity for EGFR in coelomic epithelium and a relatively weaker immunostaining in the mesonephric duct on prenatal day 13, differentiated testes and Wolffian ducts demonstrated strong EGFR immunoreactivity on prenatal day 16. Interstitial Leydig cells of the testes and Wolffian ducts showed strong immunostaining on prenatal day 18. This strong EGFR immunoreactivity was maintained during the postnatal period and remained positive in the adult epididymis, which differentiated from the proximal part of the Wolffian duct. The EGFR immunoreactivity in the gonocytes, observed on prenatal day 16, increased on prenatal day 18 and during the neonatal period, whereas it disappeared on postnatal day 10 and in adult germinal cell lines of the seminiferous tubules. It may be concluded that while EGF mediates testicular and excretory duct development and differentiation during the pre- and post-natal periods, it may also regulate the functions of testicular Leydig cells and epididymal epithelial cells in adults.