The hypertrophic rat kidney after unilateral nephrectomy showed a time-dependent increase in the RNA/DNA ratio. This was associated with the increased rate of alpha-amanitin-insensitive RNA synthesis in isolated nuclei. 2. The increased activity of solubilized RNA polymerase was due to polymerase I, II and III as judged from the DEAE-Sephadex chromatography. 3. Chromatin DNA from hypertrophic kidneys was more susceptible to DNAase I than DNA from control kidneys.