Morphine is one of the preferred drugs for the clinical treatment of pain. Both clinical and preclinical studies have reported sexual dimorphism in morphine analgesia. Different circulating levels of estrogen could be involved in sex differences in response to morphine analgesia. In our previous research, we found that capsaicin injection into the cervix of rats caused acute visceral pain that could be relieved by morphine. The role of estrogen in morphine analgesia in rats under uterine cervix pain and its underlying mechanisms remain to be explored.