Abstract Because dextromethorpan (DM) has been shown to inhibit the locomotor stimulant effects of phencyclidine (PCP), this study explored further the possible interaction between drugs acting on DM and PCP receptor sites. Caramiphen, an antitussive that binds with high affinity to the DM site, was injected (IP) alone (15–120 mg/kg) or at two doses (15 or 60 mg/kg) 15 min before a challenge dose of PCP (1.25–20 mg/kg). Caramiphen alone dose-dependently increased ataxia, increased stereotypy, and had no effect on locomotor activity. PCP alone dose-dependently increased ataxia, stereotypy, and locomotor activity, the latter showing an inverted U-shaped function. At both pretreatment doses, caramiphen enhanced locomotor activity and stereotypy when combined with low PCP doses but decreased these behaviors at high PCP doses. Caramiphen produced a dose-dependent additive effect on ataxia when combined with all PCP doses. It was concluded that, although caramiphen, like DM, inhibited the locomotor stimulant effects of selected doses of PCP, that interaction appeared to be due to other behaviors (e.g., ataxia/ stereotypy) elicited by caramiphen combined with high doses of PCP. This study underscored the importance of using full dose ranges of PCP when attempting to antagonize its behavioral effects with other drugs.