In non-immobilized male cats the stereotypy behavior evoked by threshold doses of phenamine was registered cyclographically along with the motion arrest reaction, circular reaction and their electrographic "adjuncts" (cortical caudate-induced spindles and desynchronization). Against the background of 5-Hydroxytryptophan (5-HT), a serotonine precursor, and p-chlorphenylalanine (PCPA), an inhibitor of the mediator synthesis, phenamine produced stereotype in the formerly administered dose, but its picture changed. Under the effect of 5-HT the amplitude of the stereotypy movements declined, they would become better organized, whereas after administration of PCPA they would, on the contrary, assume a chaotic "unshackled" character. The ability of phenamine to upset the behavioral and electrographic characteristics of the caudate nucleus activity was somewhat attenuated by 5-HT and potentiated by PCPA. It is suggested that the release of serotonin in the ventral region of the caudate nucleus intensified by phenamine may be responsible for the limitation of the amplitude and regulating the nature of the stereotype movements evoked by it.