6-Hydroxydopamine or 6-hydroxydopa injected systemically into newborn rats produced marked changes in the development of central and peripheral noradrenergic neurons. Noradrenaline concentration was elevated in the brain stem, particularly in the pons, and decreased in the cerebral cortex and the spinal cord while in the cerebellum, the effects were dependent on the mode of administration. The changes produced by 6-hydroxydopa in brain regional noradrenaline were related to the dose injected at birth. Similar modifications in the development of central noradrenergic neurons were found in the offspring of rats which had received 6-hydroxydopa at 16 days of gestation. The involvement of peripheral sympathetic neurons varied with the compound used and the form of its administration. Thus, 6-hydroxydopamine produced a permanent although partial peripheral sympathectomy, an effect which was less evident following multiple injections of 6-hydroxydopa after birth and almost minimal after a single injection. The prenatal administration of 6-hydroxydopa did not alter peripheral sympathetic neurons. It is concluded that with the appropriate treatment schedule, it is possible to lesion selectively the noradrenergic neurons in the central nervous system.