The role of brain monoamine systems in the jumping behavior induced by the combination of harmine and apomorphine was studied by using a MT pick-up to assess jumping behavior in rats. The jumping induced by the combination of harmine 10 mg/kg and apomorphine 2 mg/kg was enhanced by pretreatment with p-chlorophenylalanine, methysergide and by treatment with clonidine, while, it was reduced by pretreatment with 5-hydroxytryptophan, haloperidol, perphenazine, atropine and pilocarpine. The combination of harmaline or harmane and apomorphine also induced jumping with the aid of p-chlorophenylalanine. The combination of benserazide, L-DOPA and harmine induced jumping similar to that induced by harmine and apomorphine. Despite pretreatment with p-chlorophenylalanine the combination of apomorphine and benzylhydrazine, iproniazid, tranylcypromine or pargyline failed to induce jumping. These results suggest that this jumping behavior is induced not by the monoamine oxidase inhibitory effect of harmine but probably by the specific central action of harmine and on condition that the dopaminergic system is activated. The activation of this system appears to be essential, the noradrenergic system plays a facilitatory role, the serotonergic system an inhibitory role and the cholinergic system probably a specific role.