The subunit proteins p50 and p65 of the transcription factor NF-kappa B inhibitory protein were immunocytochemically identified and mapped in rat brain. The p65 subunit was localized to the cytoplasm of neurons in the lateral hypothalamus and colocalized with alpha-MSH in neurons identified as the alpha-2 component of the alpha-MSH system. The p50 subunit protein was distributed throughout the neocortex, basal ganglia, thalamic, and hypothalamic nuclei, and certain nuclei of the pons and medulla. The I-kappa B protein, which is necessary for the cytoplasmic sequestration of the NF-kappa B transcription factor complex, was identified specifically in regions of limbic, hypothalamic, and autonomic nuclei.