Although insult of the developing noradrenergic neuronal system in the brain has been associated with redistribution of noradrenergic fiber input to various target brain regions, few studies have investigated the effects of such insults on locus coeruleus cell survival. In the present study the dorsal noradrenergic bundle was transected by means of a midbrain knife cut in rats 3 days after birth, and the effects of this lesion were determined approximately 8-10 weeks later. Bymeans of an immunofluorescent histochemical procedure. it was shown that tyrosine hydroxylase-containing fibers and dopamine β-hydroxylase-containing fibers were markedly reduced in number in the neocortex and hippocampus - regions anterograde to the site of axonal transection. It was further demonstrated that the number of fluorescent fibers coursing through the dorsal bundle was similarly reduced. Sprouting of noradrenergic fibers in the brainstem and cerebellum accompanied the above alterations. When locus coeruleus cell number was determined by counting Cresyl violet-stained nucleoli in serial sections it was found that dorsal bundle transection produced a loss of 17% of the cells of the coeruleus. By dividing the counts for each nucleus into fifths, it was additionally found that approximately 20-25% of those cells comprising the midportion of the nucleus, along a rostrocaudal axis, were the ones destroyed by axonal transection. These findings indicate that a neonatal lesion of the dorsal bundle produces a loss of cells in the midportion of the nucleus locus coeruleus, and that this effect is associated with noradrenergic neuronal hyperinnervation of the brainstem and cerebellum.