The effect of cordycepin on RNA synthesis in isolated nuclei of neurones, astrocytes, and oligodendro‐ and microglial cells, is reported. This drug selectively inhibits the formation of ribosomal RNA and its precursors. Two independent components, with sedimentation coefficients of 25S and 12S, and high rate of turnover, remain however unaffected. They are shown to be non‐ribosomal RNAs and are assumed to be of messenger type. The effect of cordycepin is the same on all three types of nuclei studied.