The effects of IVT serotonin [5-hydroxytryptamine (5-HT)] and dopamine (DA) administration have been studied in rats and marmosets (Callithrix jacchus). In rats, 5-HT (114 and 170 micrograms/10 microliters) produced the same behavioral effects observed after IP administration of its precursors and agonists. The same doses of 5-HT used for rats produced only part of the behavioral effects in marmosets after IP administration of 5-HT precursors and agonists. Ataxia, vomiting, and decreased motor activity were observed, but not drowsiness or teeth-chattering. However, IVT administration of DA (400 micrograms/10 microliters dose) produced head movements or checking, ataxia, tongue out, and decreased motor activity. These findings differ from those observed after IP administration of l-DOPA and DA agonists, which increase motor activity.