Adult stem cells maintain several self-renewing systems and processes in the body, including the epidermis, hematopoiesis, intestinal epithelium, and spermatogenesis. However, studies on adult stem cells are hampered by their low numbers, lack of information about morphologic or biochemical characteristics, and absence of functional assays, except for hematopoietic and spermatogonial stem cells. We took advantage of the recently developed spermatogonial transplantation technique to analyze germ line stem cells of the rat testis. The results indicate that the stem cell concentration in rat testes is 9.5-fold higher than that in mouse testes, and spermatogenic colonies derived from rat donor testis cells are 2.75 times larger than mouse-derived colonies by 3 mo after transplantation. Therefore, the extent of spermatogenesis from rat stem cells was 26-fold greater than that from mouse stem cells at the time of recipient testis analysis. Attempts to enrich spermatogonial stem cells in rat testis populations using the experimental cryptorchid procedure were not successful, but selection by attachment to laminin-coated plates resulted in 8.5-fold enrichment. Spermatogonial stem cells are unique among adult stem cells because they pass genetic information to the next generation. The high concentration of stem cells in the rat testis and the rapid expansion of spermatogenesis after transplantation will facilitate studies on stem cell biology and the introduction of genetic modifications into the male germ line. The functional differences between spermatogonial stem cells of rat vs. mouse origin after transplantation suggest that the potential of these cells may vary greatly among species.