The effects of L-tryptophan and the major constituents of cough medicines on the antitussive effect of dihydrocodeine were examined in a comparative study with anesthetized rats. The antitussive effect of dihydrocodeine was enhanced by simultaneous administration of noscapine or methylephedrine. In rats treated with noscapine and methylephedrine together, the effect of dihydrocodeine was more markedly enhanced. Furthermore, L-tryptophan reduced by half the antitussive ED₅₀ (AtD₅₀) of dihydrocodeine. There was no difference between the AtD₅₀ of dihydrocodeine when administered in combination with noscapine and methylephedrine and that of dihydrocodeine when combined with noscapine and L-tryptophan. The AtD₅₀ of noscapine and dextromethorphan was also reduced by about half when what was administered with L-tryptophan. By contrast, the liability with respect to physical dependence on dihydrocodeine was not enhanced by the simultaneous administration of L-tryptophan. These results suggest that L-tryptophan can be considered to be a useful constituent of antitussive preparations.