The present study was designed to investigate relations of y-Glutamyl transpeptidase (y-GTP) activities to morphological differentiation and maturation of rat testicular excurrent duct system including rete testis, efferent ductuli and epididymal ducts. Animals used were Wistar rats aged from 3 days to 12 weeks. Histochemical demonstration of y-GTP activities was carried out by the method of Rutenburg et al. (1969) . Morphological differentiation of the epithelium and structure of the rete

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... s occurred between 2 and 3 weeks and completed by the age of 6 weeks, which was accompanied by induction and deletion of y-GTP activity. Morphological differentiation and maturation of the epithelium of the efferent ductuli and the epididymal ducts became evident by the age of 4 weeks and y-GTP was active in both epithelial cells throughout the period examined. Since the maturation of rete testis epithelium was found to proceed as observed as Sertoli cells, the origin and functions of the rete testis epithelium and Sertoli cells are considered to be identical. The simultaneous development of the efferent ductuli and the epididymal ducts suggests that both tissues originate from mesonephric ducts and have similar functions, although mature epithelial cells of both tissues are completely different.