A comparative study of the effects of alpha methylparatyrosine (alpha MPT) and/or reserpine pretreatments on mice motorhyperactivity and rat stereotyped behaviour induced by pyrovalerone and amphetamine suggests a different mechanism for these two substances. Both behavioral effects were abolished by alpha MPT but not altered by reserpine in the case of amphetamine which presumably acts through a selective release of "newly synthesized" catecholamines from a "functional" pool. In contrast to this, pyrovalerone would increase spontaneous motor activity through a preferential release of norepinephrine from a "storage" pool since motorhyperactivity was not altered by alpha MPT especially during the first phase, whereas it was abolished by reserpine. Stereotyped behaviour induced by pyrovalerone, was still present after pretreatment with alpha MPT or reserpine; these data suggest an action through a release of both "newly synthesized" and "stored" dopamine. On the other hand, a direct action on dopamine receptors might be involved after high doses of pyrovalerone and amphetamine since stereotyped behaviour was found to be present after a combined pretreatment with alpha MPT + reserpine.