Protein restriction has been shown to produce either an enhancement or a reduction of transcription activity in vitro. Conditions for an enhanced transcription activity were investigated. Young male rats were fed a complete diet containing either 20% or 3% casein for 6 days. Body weight changed +7.4 g/day and -0.5 g/day, respectively. Liver wet weights were 6.8 and 3.7 g and the DNA amounts/g were 1.31 and 1.67 mg. Liver nuclei were incubated without or with 1 unit micrococcus nuclease (EC 184.108.40.206) per milligram of nuclear DNA, and chromatin was fractionated into a 2,000 x g, a 102,000 x g pellet and a supernatant fraction. In the livers of rats fed a low protein diet, chromatin-bound RNA polymerase I plus III and II activity/mg of fractional and nuclear DNA and soluble RNA polymerase activity were increased, while heparin-stimulated RNA polymerase II activity remained unchanged. An increased number of chains was synthesized by RNA polymerase I plus III without change in chain length and incorporation rate per chain. The length and incorporation rate per chain increased, while the number of chains synthesized by RNA polymerase II did not. After stimulation by heparin, an increased number of short chains was synthesized at a lower rate of incorporation per chain. In the complex chromating structures the capacity for RNA synthesis was determined by specific enzyme activity, RNA chain number and length.