NF-kB is an inducible transcription factor present in many cell types in a latent cytoplasmic form. So far, only immune cells including mature B cells, thymocytes, and adherent macrophages have been reported to contain constitutively active forms of NF-kB in the nucleus. A recent study showed that the human immunodeficiency virus type 1 (HIV-1) promoter is highly active in several brain regions of transgenic mice (J. R. Corboy, J. M. Buzy, M. C. Zink, and J. E. Clements, Science 258:1804-1807,1992). Since the activity of this viral enhancer is governed mainly by two binding sites for NF-kB, we were prompted to investigate the state of NF-kB activity in neurons. Primary neuronal cultures derived from rat hippocampus and cerebral cortex showed a high constitutive expression of an HIV-1 long terminal repeat-driven luciferase reporter gene, which was primarily dependent on intact NF-kB binding sites and was abolished upon coexpression of the NF-κb-specific inhibitor IkB-α. Indirect immunofluorescence and confocal laser microscopy showed that the activity of NF-kB correlated with the presence of the NF-kB subunits p50 and RelA (p65) in nuclei of cultured neurons. NF-kB was also constitutively active in neurons in vivo. As investigated by electrophoretic mobility shift assays, constitutive NF-kB DNA-binding activity was highly enriched in fractions containing neuronal nuclei prepared from rat cerebral cortex. Nuclear NF-kb-specific immunostaining was also seen in cryosections from mouse cerebral cortex and hippocampus. Only a subset of neurons was stained. Activated NF-kB in the brain is likely to participate in normal brain function and to reflect a distinct state of neuronal activity or differentiation. Furthermore, it may explain the high level of activity of the HIV-1 enhancer in neurons, an observation potentially relevant for the etiology of the AIDS dementia complex caused by HIV infection of the central nervous system.