Megestrol acetate, a synthetic progestogen, was introduced in the early 1960s as a component in oral contraceptive preparations. In 1967, as a result of new regulations required by the United States Food and Drug Administration, megestrol acetate was submitted to long-term toxicity studies and by the early 1970s it was shown to be associated with an increased incidence of mammary tumours in beagle bitches which led to its withdrawal by several regulatory authorities. Subsequently the validity of the beagle bitch model as a predictor of carcinogenicity of steroid contraceptives has been contested by many national regulatory authorities and megestrol remains available in some countries for contraceptive purposes. In other countries its use is restricted to anticancer treatment. (Reference: (WHODI) WHO Drug Information, 1-3, 5-7, 1984)