1. Liver RNA synthesis was studied within 24h after whole-body X-irradiation of guinea pigs that had been starved for 22-24h. 2. Microsomal RNA was labelled in vivo for 3h with [(14)C]orotic acid and the isolated labelled RNA was fractionated by sucrose-density-gradient centrifugation. Incorporation was 50-100% higher between 3 and 12h after 2000rd X-irradiation and at 22h was not elevated any further. Whole nuclear RNA was labelled with [(14)C]orotic acid for 15min. At 5h after irradiation the incorporation showed a 50-100% increase. Incorporation increased in all types of RNA studied. 3. The RNA phosphorus/DNA phosphorus ratio of whole liver gradually increased after X-irradiation. Maximal increase was found between 24 and 36h, which corresponds to a value about 40% above that of the starved control. The RNA phosphorus content of isolated ribonucleoproteins obtained from various cell fractions of the liver was similarly increased after X-irradiation. 4. Liver microsomes were obtained from X-irradiated and control animals. Microsomes were incubated in vitro with [(14)C]phenylalanine in the presence and absence of polyuridylic acid. After the incubation the microsomes were fractionated by sucrose-density-gradient centrifugation. The polyuridylic acid enhancement was twice as great in the microsomes of the control preparation as in the irradiated one. The experiment demonstrated a higher saturation of microsomes by endogenous messenger after X-irradiation. 5. RNA polymerase activity of the purified nuclear preparation was assayed. The activity of the Mg(2+)-dependent RNA polymerase activity was 50 and 200% respectively above the control values at 6 and 9h after X-irradiation. 6. Animals were treated with actinomycin D shortly before X-irradiation. This treatment abolished the radiation-induced enrichment of polyribosomes and the increase of protein-synthesizing activity. The effect of X-irradiation on the transcription of the genetic code of the liver is discussed.