The concentrations of 5,7-dihydroxytryptamine (5,7-DHT) and serotonin (5-HT) were measured in brainstem, hypothalamus and cerebral cortex 0, 2, 6, 12, and 24 hours following the bilateral, lateral ventricular injection of 5,7-DHT (100 microg/each ventricle) into adult male rats. At 6 hours, 5,7-DHT levels had decreased 99% from 0 hr values in all brain regions. Thereafter, 5,7-DHT levels continued to decline in cortex, but not in hypothalamus or brainstem; at 24 hr, but not 48 hr, 5,7-DHT peaks were still measurable in each brain region examined. Serotonin levels in all three regions also fell markedly by 2-6 hours after 5,7-DHT administration. At 24 hours, hypothalamus and brainstem 5HT levels had declined >70% and cerebral cortex approximately 50% below control values. The relevance of these findings to the protective action of monoamine reuptake blockers is discussed.