Melengestrol acetate (MGA) implants were used for contraception in three addax and three Arabian oryx females housed at the Saint Louis Zoo. Serum estradiol and progesterone or fecal estrogen and progestin analysis and ultrasonography of reproductive tracts were used for monitoring changes before, during, and after MGA treatment. Follicular development and irregular uterine fluid accumulation were detected in all females during MGA treatment. Although housed with an intact male for all or most of the contraceptive period, no pregnancies occurred.One female addax may have ovulated, based on sustained elevated progesterone levels, and another showed continued follicle development, as seen by fluctuating estradiol concentrations. Reversibility was documented in two of the three addax that resumed reproductive cycles post-MGA-implant removal, whereas the third, a peripubertal female, did not cycle before, during, or after treatment. Addax females were lost to further follow-up after transfer to another institution, so the possibility of subsequent pregnancies is not known.All three Arabian oryx ovulated during the initial MGA treatment, but two of the three females had implants past the typical 2-year efficacy period. They had regular ovulatory cycles after implant removal, with mean cycle length of 27.5+/-1.5 days and mean luteal phase duration of 15.2+/-0.7 days. Reversibility was further shown in all three oryx by pregnancies after placement with a male approximately 2 years after MGA implant removal. Two produced healthy calves, but the third died owing to an unrelated terminal illness in the mother. Zoo Biol 26:299-310, 2007. (c) 2007 Wiley-Liss, Inc.