The incubation of isolated nuclei obtained from 10-day-old mouse brain in the presence of brain cell cytosol resulted in an increase in the synthesis of RNA. Under conditions of saturating concentrations of nucleoside triphosphates, the influence of cytosol could not be duplicated by the addition of cyclic nucleotides. The stimulatory activity of cytosol on brain nuclear RNA synthesis could not be attributed to either alterations in the permeability of the nuclear envelope or an increased uptake of radioactively-labeled precursors. Sucrose gradient analysis demonstrated that the RNA products synthesized by nuclei isolated from 10-day-old and adult mouse brain were of a relatively low molecular weight. However, the addition of cytosol resulted in a significant increase in the size of the RNA transcripts. In contrast to the observations with 10-day-old and adult brain nuclei, the RNA from 2-day-old mouse brain nuclei was larger in size and relatively unaffected by the presence of cytosol. Although cytosol caused an increase in the amounts of poly[A]-RNA in nuclei of 2-day-old and adult animals, no comparable effect could be measured in nuclei from 10-day-old brain tissue.