Previous work has demonstrated that direct injections of methylnaloxonium (MN), a hydrophilic quaternary opiate antagonist, in the area of the nucleus raphe pontis (RPn) significantly attenuated alfentanil-induced muscle rigidity in the rat. To extend these observations and to explore further the regions important for opiate-induced rigidity, rats were implanted with chronic guide cannulae aimed at discrete brain sites with an emphasis on the region from the periaqueductal grey (PAG) to the RPn. Each animal was pretreated by a blinded observer with an intracerebral injection of MN (125 ng total dose) or saline, and electromyographic (EMG) activity was recorded from the gastrocnemius muscle. Alfentanil (ALF; 500 micrograms/kg) was then administered subcutaneously and the magnitude of tonic EMG activity was assessed as a measure of hindlimb rigidity. The administration of MN into the pontine raphe nucleus (RPn) and also into the more lateral nucleus reticularis tegmenti pontis significantly attenuated ALF rigidity compared with saline-pretreated controls. Within the midbrain, MN selectively reversed rigidity when injected into the periaqueductal grey (PAG). The dorsal PAG appeared to be a more important site than the ventral PAG. There was no significant effect on ALF rigidity of MN injections into brain regions between the ventral PAG and the RPn while MN injections into the deep layers of the superior colliculus, lateral to the dorsal PAG, partially attenuated ALF rigidity. In contrast, rigidity was not consistently reversed after MN injections into the basal ganglia, the dorsal superior colliculus, or the region of the decussation of the dorsal tegmentum. This study provides strong evidence that nuclei of the reticular formation, specifically the PAG, raphe pontis, and reticularis tegmenti pontis that are known to play a role in other opioid-mediated behaviors, are important in opiate-induced muscle rigidity in the rat. These results could have implications for the prevention of this undesirable effect of high-dose opiate administration.