Technical Abstract: Initial studies suggested that Siberian tiger gonadotropins are more homologous with those of porcine than any other commercially available preparation. Therefore the present study measured the efficacy of repeated ovarian stimulation with purified porcine gonadotropins on the follicular, hormonal and immunogenic responses in Siberian tigers. In addition, the ability of oocytes retrieved by laparoscopic follicular aspiration to fertilize and cleave in vitro was evaluated. Controlled rate and vitrification cryopreservation methods were also compared for their ability to support ongoing cleavage following thawing of presumptive tiger embryos generated in vitro. Vitrification supported continued embryonic cleavage in vitro while controlled rate freezing did not. Stereological microscopy indicated an excellent ovarian response with the recovery of quality cumulus-oocyte complexes that apparently fertilized and cleaved in vitro. However, closer examination of the results at the ultrastructural and physiological levels revealed abnormal and unnatural responses such as the failure of cumulus-oocyte complexes to reach maturity and hormone levels to approach normalcy. At the same time, analyses of blood for antibodies failed to detect an immune reaction to these foreign gonadotropins in an assay that tested positive for the pLH-challenged domestic cat. Together, these observations indicate that while the use of porcine hormones for ovarian stimulation in the Siberian tiger has value, some limitations to the approach still exist. These data support research in progress to develop homologous gonadotropins for assisted reproduction efforts in endangered felids.