It has been proposed repeatedly that the noradrenergic (NE) system may exert an influence on cortical development. We have tested this proposition by examining synaptogenesis in the visual cortex of rats whose NE afferents were selectively lesioned by injections of the neurotoxin 6-hydroxydopamine (6-OHDA). Control littermates were injected with equal volumes of vehicle. Montages of electron micrographs covering approximately 50 micrometers-wide strips of cortex were assembled from both groups of animals at 2,4,6,8,14, and 90 days of age. Synapse counts revealed a significantly higher density of synapses in the cortex of 6-OHDA-treated rats during the first week of postnatal life. The difference between the experimental and control rats was less apparent during the second postnatal week, and at day 90 the densities of synapses were similar for the two groups of animals. The enhanced density, which was the result of the increased number of Gray's type I synapses, was confined to the subplate region at day 2 but became more widespread in the cortex at subsequent stages of development. From these observations it would appear that the NE system exerts an inhibitory influence on synapse formation in the visual cortex in early postnatal life.