Estrogenic activity of norethindrone in the immature female beagle.

Abstract

The daily oral administration of 12.8 mg/kg of norethindrone for 30 days in immature female beagle dogs caused clinical signs of approaching estrus. At necropsy the uterus cervix and vagina were enlarged. Microscopically the uterus had moderate simple glandular proliferation and the cervix and vagina were lined by proliferative squamous epithelium typical of estrogenic stimulation. The predominance of estrogenic stimulation after treatment with norethindrone may be responsible for the lack of progestogen-related mammary changes in dogs involved in long-term oral contraceptive steroid studies. (authors)