Serotonergic neurons throughout the brain were destroyed by early postnatal treatment of rats with an intracisternal injection of 5,7-dihydroxytryptamine (5,7-DHT), as demonstrated with biochemical measurements of serotonin and immunocytochemical localization of serotonin-containing neurons. Using these methods, it was shown that approximately 75-98% of serotonergic neurons underwent cell death in rats which were treated on day 3. In contrast, intracisternal administration of 5,7-DHT in adult rats led to the loss of distal serotonergic terminals without apparent loss of the cell bodies. Desipramine pretreatment prevented significant effects of 5,7-DHT on noradrenergic neurons.