We present a model of in vivo testis transplantation in order to study the extent of physiological production of testosterone when Leydig cells are transplated to the objects with deficiency of testosterone due to secondary or primary gonadal dysfunction.Twenty two rats of S-6 weeks were first castrated. After ten days, testes of neonate rats or of full-term fetal rats were transplanted into the bilateral subrenal capsule of the castrated rats under the operating microscopy. Fifteen rats that survived were divided into three groups, 2-week, 4-week, and 6-week groups (5 rats each) according to the time elapsed after transplantation before they were sacrificed. As the transplantation period went by, there was a significant increase in serum testosterone level and the sizeof seminal vesicle became larger. There was a significant difference in the seminiferous tubules of 2-, 4-, and 6-week groups and corresponding normal group: a lot more of early stage sperma tocyte were found in the normal 2-week group than the 2-week group of transplanted rats. Also, the spermatid that were found in normal 4- and 6- week control groups were not present in the corresponding testes transplante groups. In order to differentiate two developmental phase of Leydig cell and to understand the func-tional differences, a further study of ultrastructural analysis of Leydig cell and interactions of different cells in the tissues of testis would be necessary.