The present study was performed to optimize a superovulation protocol in rats in order to produce a large number of good-quality embryos suitable to develop rat embryonic stem (rES) cells. We first evaluated the ovulation kinetics of three rat strains: Wistar, Fisher and ACI/N. Animals (n=30 per strain) were treated with 50 IU of pregnant mare serum gonadotrophin (PMSG), and ovulation was induced with 50 IU of human chorionic gonadotrophin (hCG) 50 h apart. Next, we evaluated the dose-response curves of PMSG and hCG in Wistar rats in order to obtain the highest number of embryos. The parameters evaluated for superovulation efficiency were: percentage of mated females, percentage of pregnant females and the average number of embryos collected per female. The results of these experiments suggested that the best dose combination was 50 IU for each hormone. Subsequent experiments, again with Wistar rats, were designed to test which of four hormonal combination treatments (30/30, 30/50, 50/30, and 50/50 IU of PMSG/hCG) will produce the largest numbers of good-quality embryos. Embryo quality was evaluated by embryo development uniformity, embryo morphology, embryo survival in an in vitro culture and embryo ability to generate rES-like cells. Results from these experiments showed that 30/50 IU of PMSG/hCG was the treatment that induced the best embryo quality. In conclusion, our results indicated that, in Wistar rats, the most appropriate hormonal combination dose for superovulation protocols with high number of good-quality embryos was 30 IU of PMSG and 50 IU of hCG given 50 h apart. We are performing further studies with rES-like cells produced with the present methodology to evaluate if they are able to participate in the production of germ-line chimeras.