Chronic painful temporomandibular joint disorders (TMD) occur more often in women than men and are difficult to manage. Sex difference in kappa opioid analgesia have been reported under a variety of test conditions in animal and human studies; however, the influence of estrogen (E2) on kappa opioid receptor (KOR) of nociceptive processing related to craniofacial pain is not well defined. To determine if estrogen mediate the variation in kappa opioid analgesia of TMJ unit in female, single units were recorded in laminae I-II at the spinomedullary (Vc/C1-2) junction from ovariectomied female rats (OvX) treated with high E2 (40μg/d, HE2), low E2 (4μg/d, LE2). Under isoflurane anesthesia TMJ units were activated by ATP (1 mM, 20μl) injected into the joint space before and during cumulative doses of U50,488H (selective KOR agonist; 0.03- 3mg/kg, iv ) given at 20 min intervals. ATP-evoked responses of TMJ units in HE2 were enhanced versus LE2 (p<0.05). U50,488H caused a doses related inhibition of ATP-evoked unit activity in HE2 rats, while units in LE2 rats displayed inconsistent effects. BNI (selective KOR antagonist; 1mg/kg) caused at least partial reversal of kappa opioid inhibition. These results suggest that estrogen status plays a significant role in kappa opioid analgesia of TMJ units.