Consistent with their antagonistic actions at N-methyl-D-aspartate type glutamate receptors, dextromethorphan (DXM) and its metabolite, dextrorphan (DXT) decrease the intensity of opioid withdrawal syndrome. Since quinidine (QND) affects CYP2D6-mediated metabolism and P-glycoprotein governed transport, we sought to determine whether co-treatment with QND would affect brain levels of DXM and DXT as well as the effect of these compounds on opioid withdrawal syndrome in mice. We found that DXM dose dependently inhibited the intensity of opioid withdrawal syndrome and that there was a tendency for a further decrease when QND was co-administered with DXM. Administration of 30 mg/kg of DXM resulted in higher brain levels of DXM and DXT than administration of 10 mg/kg of DXM, but much lower DXT levels than that produced by 30 mg/kg of DXT. Co-treatment with QND resulted in higher brain levels of DXM (but not DXT) suggesting that QND produces an increase in the brain availability of DXM. In summary, brain levels of DXM were inversely correlated with the intensity of opioid withdrawal syndrome. QND induced increased brain levels of DXM tend to attenuate the intensity of opioid withdrawal syndrome. We suggest that it is DXM, rather than DXT, that is responsible for the attenuating effect on the intensity of opioid withdrawal syndrome, and that the beneficial action of QND on the effect of DXM should be more pronounced in humans.