Colonies of naked mole-rats (Heterocephalus glaber) contain a single dominant, breeding female, or "queen", which suppresses reproduction in subordinate females. In this study, the queen and breeding male were removed from a colony at Brookfield Zoo. We examined behavioral and endocrine changes in the remaining colony females. Behavioral observations were conducted weekly and urine samples were collected beginning 3 months prior to the planned removal of animals. Prior to the removal, only the queen displayed the high frequencies of aggressive shoving typical of breeding females. In the first 2 months following removal of the queen, three non-breeding females markedly increased their frequencies of shoving. Urinary progesterone showed that one of three females had probably ovulated both before and after queen removal. Histological examination following fatal agonistic interactions confirmed ovulatory function in two of these three females. Similar behavioral and endocrinological results were obtained for three more females, following the combat deaths of the first three females. These findings suggest that ovarian activation facilitates intrasexual aggression in female naked mole-rats and might contribute to attainment of breeding status.