Reversible contraceptives, such as melengestrol acetate (MGA) implants, have become an invaluable tool that captive-animal managers use to maintain various species in social groupings while avoiding unwanted pregnancies. The American Zoo and Aquarium Contraception Advisory Group (AZA
CAG) monitors the efficacy, reversibility, and safety of contraceptives used in captive exotic mammals worldwide. Because so few data exist on contraceptive efficacy and safety in exotic species, it is critical that evaluations reflect an understanding of the mechanism of action of the active
ingredient as well as of the delivery system. The following discussion addresses the concerns of the CAG over the manner in which De Vleeschouwer et al (Animal Welfare 2000, 9: 251-271) analysed MGA implant reversibility data in golden-headed lion tamarins.