['Although the uterine cervix is a common source of acute and chronic visceral pain in women, there is practically no neurobiological investigation of nociception from this visceral organ. With use of a novel model of uterine cervical distension nociception in rats, the estrogen dependency of opioid agonist-induced inhibition was investigated.', 'Sprague Dawley rats were anesthetized with halothane and bilateral ovariectomy was performed, after which placebo or estrogen treatment was administered for 1 week. Animals were reanesthetized and fine metal rods were inserted into the uterine cervix for manual distension. Reflex contraction of the rectus abdominis in response to distension was recorded before and after cumulative dosing with the mu-opioid agonist morphine and the kappa-opioid agonist (-)U50488.', 'Uterine cervical distension increased reflex abdominal muscle contraction with a threshold of 75 g, regardless of estrogen treatment. Morphine and (-)U50488 reduced the reflex response to cervical distension in a dose-dependent manner. Estrogen reduced the inhibitory effect of morphine but not that of (-)U50488.'] Does estrogen reduce efficacy of mu- but Not kappa-opioid agonist inhibition in response to uterine cervical distension?