Experiments were performed to address the known relationship between diet and longevity. The acute response of rat liver nuclear RNA polymerases to inhibition of protein synthesis by cycloheximide (10 mg/kg) was quantitated in rats of varying ages. Thirty minutes after cycloheximide administration, there was a "compensatory" 1.5--2-fold increase in RNA polymerase II activity at all ages. Nucleolar RNA polymerase I activity was significantly diminished only up to 4 months, indicating a loss in tight control coupling of protein synthesis to rRNA synthesis with age.