—Transcription of repeated and unique DNA in rat brain nuclei was examined using a RNA-DNA hybridization technique. Saturation hybridization experiments with a membrane filter showed that there were differences in the base sequence complements between labelled brain and labelled liver nuclear RNA. In competition hybridization experiments using a membrane filter, brain nuclear labelled RNA was in competition most effectively with unlabelled brain nuclear RNA, but less effectively with kidney or liver nuclear RNA. Experiments using a membrane filter loaded with isolated unique or repeated DNA fragments gave similar results. Furthermore, competition hybridization with a DNA excess also showed similar results at Cot 10,000. These results indicate the existence of tissue-specificity in the transcription of unique and repeated DNA sequences in brain nuclei. Subsequently, rat brain nuclei were separated into neuronal and glial nuclear fractions for the purpose of detecting the cell-specificity of RNA. The results of these experiments give evidence of significant differences in the transcription of unique DNA sequences, but indicate no differences in that of repeated DNA sequences between the two nuclear classes.