The recent reports on the treatment of azoospermia patients, in which spermatozoa could not be traced in their testes, are focused more on the potential use of adult stem cells, like mesenchymal stem cells (MSCs). The aim of this study was to demonstrate the potential use of MSCs derived from adipose tissue in the treatment of azoospermia using rat disease models. After left rete testes. After 12 weeks, the testes with cell injection (right testes) were compared to control (left testes) after dimensional and immunohistochemical analyses. Testes treated with MSCs appeared morphologically normal, but they were atrophic in rats without stem cell treatment, in which the seminiferous tubules were empty. Spermatogenesis was detected, not in every but in some tubules of cell-treated testes. GFP(+)/VASA(+) and GFP(+)/SCP1(+) cells in testes indicated the transdifferentiation of MSCs into spermatogenetic cells in the appropriate microenvironment. Rats with cell treatment were mated to show the full recovery of spermatogenesis, and continuous generations were obtained. The expression of GFP was detected in the mesenchymal stem cells derived from adipose tissue and bone marrow and also in the sperms of offspring. In conclusion, MSCs might be studied for the same purpose in humans in future.