The effect of Chlorimipramine and Muscimol on serotonergic activity in rat cerebral arteries and in dorsal and median raphe nuclei were used to study the presence of a serotonergic innervation in the cerebral blood vessels functionally dependent on the brainstem nuclei activity. Serotonergic activity was appraised in rat cerebral arteries from 5-hydroxyindoleacetic acid (5-HIAA) disappearance rate or 5-hydroxytryptophan (5-HTP) accumulation after inhibiting monoamine oxidase (MAO) or aromatic amino acids decarboxylase, respectively. In dorsal and median raphe nuclei the decay with time of 5-HIAA after MAO inhibition was used to estimate serotonergic activity. Chlorimipramine significantly reduced serotonergic activity in cerebral blood vessels and in both raphe nuclei. 5-HIAA basal levels in these blood vessels were not altered by treatment with the drug. Muscimol evoked only a decrease in the serotonergic activity of the median raphe nucleus. These results suggest that rat cerebral arteries receive serotonergic fibers functionally active arising mainly from dorsal raphe nucleus.