The author carried out a study on the ultrastructure of germinal and Sertoli's column in sexual immature tests of 21 rats of Wistar strain, examined dynamically into the following age groups: newborns on the fifth and tenth days after delivery. After analysing the obtained results the following conclusion could be made: The lack of specific cellular sign of gonocyte to differential A-spermatogonia was a serious obstacle in the study of prepubertal development of male gametogenesis. Examining the differentiation of cellular populations in sexual immature testes of rats, the dynamic of proliferation and ultrastructural changes in the germinative Sertoli's cells, which transformed gradually into their specific structure of sexual mature tests, was impressive. Although the rather difficult determination of factors, coordinating interrelationships between these cells, should be taken into consideration, it is clear that the development of germinal cells could be described always in close connection with the surrounding Sertoli's cells, which play probably a basic role in initiation of their proliferation and entry in miosis.