male immunodeficient mouse model for transplantation of ovarian tissue was investigated. Bovine and human ovarian tissues were surgically placed either under the kidney capsule or in the subcutaneous spaces of male non obese diabetic (NOD) severe combined immunodeficient (SCID) mice. Time intervals required for development of growing follicles were determined for neonatal and adult bovine ovarian tissue grafts. This interval was much shorter (P <0.01) in adult tissue than in one-week-old calf tissue, i.e. 55 vs 124 days. The increase in the proportion of growing follicles was coincidental with a decrease in the proportion of resting follicles. This increment in the growing follicle populations took place abruptly and was significant by 55 days and by 124 days after transplantation in the adult cow and calf ovarian grafts, respectively. Recovery of oocytes from bovine ovarian grafts was successful. Several immature oocytes were recovered and evidence of maturation in one oocyte was obtained after 24 hours of in vitro maturation. Treatment of host mice with an FSH:LH preparation increased follicular development but did not enhance oocyte recovery rates. Human ovarian tissue grafted under the kidney capsule of intact male NOD SCID mice showed a greater proportion of growing follicles than similar grafts transplanted to the kidney of castrated hosts and to the subcutaneous space of intact hosts. However, no differences in follicular growth and development were detected between the intact/ kidney capsule and the castrated / subcutaneous groups. It is clear that the site of transplantation and presence or absence of the hosts gonads can potentially influence follicular development in xenotransplanted ovarian tissue. The male NOD SCID mouse model is a feasible alternative for xenotransplantation of mammalian ovarian tissue. This model should prove useful as a tool for further development of strategies for preservation of fertility of young female cancer patients and endangered species.