Adult rats were subjected to chronic treatment with the cannabinoid agonist, Delta9-tetrahydrocannabinol, or with vehicle, and their brains used to analyze the contents of serotonin (5HT) and of its intraneuronal metabolite, 5-hydroxyindolacetic acid (5HIAA). 5HT and 5HIAA contents were not affected by chronic cannabinoid administration in most of the brain regions analyzed. We found a marked increase in 5HT contents in the frontal cortex that was accompanied by no changes in 5HIAA contents. This originated a decrease in 5HIAA/5HT ratio, which suggests a possible reduction in the activity of serotoninergic terminals reaching this cortical area. This effect was not seen after an acute injection of this cannabinoid. The relevance of these observations was that they occurred in a region where changes in serotoninergic transmission have been implicated in the development of depression; therefore, our data support the theory that the cannabinoid system might be a potential target for the treatment of this neuropsychiatric disease.