The proper growth and elaboration of neural processes is essential for the establishment of a functional nervous system during development and is an integral feature of neural plasticity throughout life. Nuclear factor-kappa B (NF-kappa B) is classically known for its ubiquitous roles in inflammation, immune and stress-related responses and regulation of cell survival in all tissues, including the nervous system. NF-kappa B participation in other cellular processes remains poorly understood. Here we report a mechanism for controlling the growth of neural processes in developing peripheral and central neurons involving the transcription factor NF-kappa B. Inhibiting NF-kappa B activation with super-repressor I kappa B-alpha, BAY 11 7082 (I kappa B-alpha phosphorylation inhibitor) or N-acetyl-Leu-Leu-norleucinal (proteosomal degradation inhibitor), or inhibiting NF-kappa B transcriptional activity with kappa B decoy DNA substantially reduced the size and complexity of the neurite arbors of sensory neurons cultured with brain-derived neurotrophic factor while having no effect on their survival. NF-kappa B exerted this effect during a restricted period of development following the phase of naturally occurring neuronal death when the processes and connections of the remaining neurons are extensively modified and refined. Inhibiting NF-kappa B activation or NF-kappa B transcriptional activity in layer 2 pyramidal neurons in postnatal somatosensory cortical slices reduced dendritic arbor size and complexity. This function of NF-kappa B has important implications for neural development and may provide an explanation for reported involvement of NF-kappa B in learning and memory.