In the present study we assessed by LC/MS method the influence of amantadine and dextromethorphan on morphine metabolism in humans in order to explain clinically observed phenomenon of amplification of analgesic action of morphine by these drugs. Neither dextromethorphan nor amantadine influenced the rate of morphine degradation and concentration of morphine metabolites. Thus, our results suggest pharmacodynamical, not pharmacokinetic mechanism of interaction.