THIS study assessed the effect of peripherally applied opioids on the electromyographic activity reflexly evoked in digastric and masseter muscles by injection of the small-fiber excitant and inflammatory irritant mustard oil (MO) into the temporomandibular joint. In 39 anaesthetized rats, local pretreatment of joint tissues with morphine (15 nmol) significantly depressed the jaw muscle responses compared with saline, and the depression was antagonized by simultaneous local injection of the opiate antagonist naloxone (2.7 nmol); systemic morphine pretreatment (15 nmol, i.v.) did not influence the muscle responses. The naloxone-reversible depression of the MO-evoked muscle responses by local, but not systemic morphine, supports the presence of peripheral opioid receptors that may have a role in modulating nociceptive responses.