Abstract Twenty anestrus bitches were dosed orally for 90 days with vehicle or 0.9 mg/kg b.w. of 6,6-spiroethylene-19-nor-spiroxenone. All dogs were due in estrus during the test period based on previous average estrous cycle lengths. Estrus was prevented in 100% of the drug-treated bitches. Eight of ten control dogs came into heat during the test period. Hematological and blood biochemical studies did not show any consistent drug-related anomalies. Uterine biopsies, taken at laparotomy from half the dogs in each group, showed no evidence of cystic endometrial hyperplasia or pyometra. Mammary biopsies were also normal. Functional corpora lutea were observed in two control dogs at laparotomy. These dogs had high serum progesterone levels and a secretory endometrium. Drug-treated dogs had low serum progesterone levels and quiescent ovaries.