Intracisternal administration of 5,7-dihydroxytryptamine (5,7-DHT) to immature rats produced a marked reduction of brain norepinephrine and serotonin. Accompanying these reductions of brain amines were a decrease in body weight and alterations in behavior. After treatment with 5,7-DHT at 3 days of age, locomotor activity was significantly elevated at 14 days of age, but was reduced at 28 days of age. 5,7-DHT alone also induced a significant deficit in acquisition of the shuttle-box avoidance response and blocked body movements observed after decapitation. Treatment with either pargyline or desipramine before injection of 5,7-DHT eliminated the effect of 5,7-DHT on noradrenergic fibers while enhancing the effects of 5,7-DHT on brain serotonergic fibers. These treatments also minimized the deficits of 5,7-DHT on growth and on acquisition of the avoidance response and reversed the blockade of decapitation convulsions. However, animals pretreated with pargyline or desipramine before they received 5,7-DHT still demonstrated hyperactivity at 14 days of age equivalent to that observed in neonates that received only 5,7-DHT. Furthermore, a behavioral syndrome induced by 30 mg/kg of 5-hydroxytryptamine was markedly potentiated by all of the 5,7-DHT treatments which suggest that serotonin receptors were supersensitive.