Pretreatment with L-histidine, a precursor of brain histamine, and promethazine, a H₁ receptor blocker, failed to modify apomorphine-induced stereotyped behaviour in rats. In contrast, pretreatment with L-histidine significantly decreased the intensity of amantadine stereotypy while pretreatment with promethazine significantly increased the intensity of amantadine stereotypy in rats. The results suggest that drugs which influence central histaminergic mechanisms are effective only in modifying the stereotyped behaviour induced by the indirectly-acting DA agonist amantadine, and fail to modify the stereotyped behaviour induced by apomorphine, a directly-acting DA agonist.