Embryo autotransfer is defined as the collection of an embryo from and the transfer of this embryo into the same animal. The objectives of this study were to: 1) test the hypothesis that oviduct transport of the equine embryo from the oviduct into the uterus is not dependent on a unilateral embryo-corpus luteum interaction, 2) develop an embryo autotransfer technique for the mare and 3) compare the success rates of Day 4 embryos surgically autotransferred from the oviduct ipsilateral to ovulation to either the oviduct (n=10 mares) or the uterine horn (n=10 mares) contralateral to ovulation. Seventy percent (7 10 ) of the Day 4 embryos which were autotransferred to the oviduct contralateral to ovulation were transported through the oviduct and subsequently developed into embryonic vesicles detectable by ultrasonography between 10 and 21 days postovulation. This finding supported the hypothesis that oviductal embryo transport is not dependent upon the ipsilateral corpus luteum. Overall, sixty percent (12 20 ) of the autotransfers were successful. The success rate of uterine-transferred embryos was not significantly less (P>0.3) than that of oviductal-transferred embryos (5 10 vs 7 10 , respectively). Therefore, the Day 4 equine embryos were apparently mature enough to survive in the mare's uterus.