By continuous perfusion of columns containing isolated immobilized rat liver nuclei with media containing labeled RNA precursors, the in vitro synthesis and release of RNA was studied. The combined reaction of synthesis and release could be adjusted to proceed at a constant rate. The reaction rate responded to variation of termperature, ionic conditions, nucleoside triphosphate concentration and to the addition of RNA polymerase inhibitors. During 60 min perfusion approximately equal amounts of radioactive low molecular weight RNA and of ribonucleoproteins were released. Pulse-chase experiments showed that the low molecular weight RNA was synthesized throughout the perfusion and released immediately after formation. The ribonucleoproteins were primarly labeled during the first period of perfusion and were gradually released. Synthesis of RNA contained in the ribonucleoproteins was inhibited by low alpha-amanitin concentrations, indicating that it was catalyzed by RNA polymerase II. The in vitro labeled ribonucleoproteins exhibited properties of the stable nuclear particles which can be extracted from isolated nuclei after rapid in vivo labeling of RNA. They had a buoyant density of 1.41--1.43 in CsCl, were partially unstable in 1% deoxycholate, but stable in 0.1% deoxycholate, in 100 mM NaCl and in 10 mM EDTA. Due to the dilution by the perfusion medium, the ribonucleoproteins sedimented with a peak at 22--27 S, and not at 30--45 S. The RNA synthesized in the immobilized nuclei was not degraded during the perfusion. Less than 20% was gradually released, whereby the 20--30 S peak zone was reduced. While the properties of the in vitro labeled ribonucleoproteins and of rapidly in vivo labeled ribonucleoproteins were the same, the kinetics of their release differed.