Dextromethorphan (DM), a non-narcotic antitussive, binds in the guinea pig brain to specific high- and low-affinity sites with Kd values of 57 nM and 24 microM, respectively (Musacchio et al., 1988). The antitussives carbetapentane, caramiphen, butamirate and dimethoxanate competed with the high-affinity binding of [3H]DM at pH 7.4 with nanomolar Ki values. Sigma site ligands showed high affinity for [3H]DM binding sites. The rank order of potency was: haloperidol greater than (+)-pentazocine greater than (+)-cyclazocine greater than 3-(-3-hydroxyphenyl)-N-(1-propyl)piperidine greater than (+)-N-allylnormetazocine greater than (-)-butaclamol much greater than (+)-butaclamol (-)-N-allylnormetazocine. The antipsychotic perphenazine competed with low nanomolar Ki values, whereas rimcazole was weaker. The antidepressant opipramol and the benzomorphan (+)2'methoxyphenazocine were the most effective drugs tested, with Ki values of 0.4 nM. By contrast, MK-801 and phencyclidine hydrochloride were very weak competitors for [3H]DM binding. The diphenylalkylamines were the most effective competitors of the calcium channel blocking agents: prenylamine and cinnarizine had Ki values of 17 and 22 nM, respectively. Lidoflazine and hydroxyzine were slightly less potent, but nifedipine and the benzothiazepine diltiazem were much weaker. Potassium channel blockers inhibited DM binding in pharmacologically relevant concentrations: primaquine was the most effective with a Ki of 0.5 microM. Other antimalarial potassium channel blockers tested inhibited binding in the micromolar range. 4-Aminopyridine and tetraethylammonium had Ki values of 0.76 and 1.40 mM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)