Alfentanil is a potent and short-acting mu opioid agonist that produces both antinociceptive effects and muscle rigidity. In the present study, the susceptibility of alfentanil-induced antinociception and rigidity to antagonism by the selective mu antagonist beta-funaltrexamine and the selective mu-1 antagonist naloxonazine was examined. Alfentanil (37.7-150.0 micrograms/kg) produced a dose-dependent increase both in antinociception as measured by the warm-water tail-dip assay and in rigidity as measured by electromyographic recording of the gastrocnemius muscle. Both beta-funaltrexamine (10.0 and 20.0 mg/kg) and naloxonazine (7.5 and 15.0 mg/kg) produced dose-dependent and parallel rightward shifts in the alfentanil dose-effect curves for both antinociception and rigidity. Furthermore, the alfentanil dose-effect curves for antinociception and rigidity were shifted to the right to a similar degree by any given pretreatment. These results suggest that alfentanil-induced antinociception in the warm-water tail-dip test and rigidity are mediated by pharmacologically similar populations of opioid receptors. More specifically, these results suggest that mu-1 opioid receptors mediate both alfentanil-induced antinociception and rigidity.