The effect on the developing cerebral cortex and hippocampus of removing the noradrenergic input was investigated in 3-month-old rats following unilateral electrolytic lesions performed semistereotaxically at 15 h postnatal. In 5 animals there was a significant reduction (mean = 74%) in noradrenaline in the cortex ipsilateral to the lesion, and the neuronal morphology on the two sides was studied with the Golgi technique. There was no apparent difference in the morphology of pyramidal cells in parietal cortex with respect to soma depth, apical dendritic length, number of basal dendrites or number of spines on selected dendritic regions. There was a small but significant increase in the number of dendritic branches of pyramidal cells in layers II and IV on the side ipsilateral to the lesion. No layer VI cells were identified which had retained their contact with layer I. An examination of the cell morphology of CA1 pyramidal and dentate granule cells in the hippocampus revealed no obvious differences between the two sides. These results suggest that the trophic influence of the noradrenergic innervation on the postnatal development of cells in the cerebral cortex and hippocampus, if it exists at all, is relatively minor.