Unilateral ligation of a common carotid artery in gerbils causes a decreased rate of serotonin synthesis and degradation but an increased release of this monoamine. In the brain, reduction of cerebral serotonin content during ischemia is followed by accumulation of its main metabolite, 5-hydroxyindolacetic acid. These data support the contention that serotonin plays an important role in the progression of cerebral infarction.