Nuclear factor-kappa B (NF-kB) transcriptional activity is induced by numerous stimuli. To identify tissues exhibiting NF-kB transcriptional activity during development, we analyzed transgenic reporter mice that express β-galactosidase from an NF-kB–responsive element. We report that NF-kB activation is widespread and present in numerous epithelial structures and within vasculature. Several regions of the developing central nervous system, including the roof plate and floor plate of the midbrain, show prominent NF-kB activation. To assess the role of the TRAF6 adaptor protein in developmental NF-kB activity, we analyzed NF-kB activation in reporter mice rendered null for TRAF6. Deletion of TRAF6 resulted in the loss of NF-kB activity in epithelia, in vasculature, and in roof and floor plate but had no effect on NF-kB activity developing telencephalon, choroid plexus, cochlear canal, and thymus. These data indicate that NF-kB transcriptional activity is present in a broad range of structures during development and that TRAF6 plays a critical role mediating developmental NF-kB activation in many but not all tissues. Developmental Dynamics 231:122–127, 2004. © 2004 Wiley-Liss, Inc.