Nuclear factor-kappa B (NF-ĸB) is a transcription factor, which is ubiquitously distributed in the adult nervous system and is thought to be crucially involved in the neuronal and glial functions. However, during development, the spatiotemporal expression pattern of NF-ĸB has not been well characterized, especially in developing primates and humans. In the present study, we used an immunohistochemical method to determine the distribution of NF-ĸB in the developing rhesus monkey (Macaca mulatta) brain at different developmental stages. Immunoreactive products were not observed at 25 days of pregnancy but were detected at 40 days mainly in the perinuclear region, and then, gradually more positive signals were found inside the nucleus of cells in different brain regions. At this stage, when various primitive brain structures had just started to form, a low level of immunoreactivity was detected in the presumptive areas of the thalamus, hypothalamus, caudate nucleus and parietal cortex. The immunoreactive cells were scattered and large, usually with a prominent nucleus. In the developing choroid plexus, moderate immunoreactivity was observed in the epithelial lining of the ventricle and also in the underlying mesenchymal tissues. By 55 days, immunoreactivity became stronger, and brain regions were more easily identified. Both the number of the immunoreactive cells and the intensity of positive signals were increased, and cells were more differentiated. Some of these cells extended cytoplasmic processes. Positive cells were also scattered without any specific pattern of distribution in all the brain regions examined. The columnar epithelium of the choroid plexus showed intense immunoreactivity. Our findings show that NF-ĸB starts to express in many regions of the early developing monkey brain, and the immunoreactivity of NF-ĸB increases with time.