The profile of action of eptazocine, a novel analgesic, on opioid receptors was investigated. Eptazocine caused a concentration-dependent inhibition against the [3H]-naloxone [( 3H]-NLX) specific binding to rat brain synaptic membrane in the absence of sodium cation and GTP (IC50; 7.83 +/- 1.57 microM). The ratios of IC50 values between the absence to the presence of sodium cation alone or sodium cation and GTP were 3.89 and 4.35, respectively. In addition, eptazocine (10 microM) also produced the significant decrease of [3H]-NLX specific binding in the mouse brain synaptic membrane. Moreover, the same dose eptazocine significantly decreased the [3H]-ethylketocyclazocine [( 3H]EKC) specific binding, but not [3H]-phencyclidine [( 3H]-PCP). These results suggest that eptazocine interacts with opioid receptor, and is classified as one of the opiate agonist-antagonist analgesics.