The ribonucleoprotein particles released from isolated nuclei of regenerating rat liver in two in vitro systems were studied and the following results were obtained. 1. When the isolated nuclei of regenerating rat liver labeled in vivo with [14C] orotic acid were incubated in medium containing ATP and an energy-regenerating system (medium I) release of labeled 40-S particles was observed. Analysis of these 40-S particles showed that they contained heterogeneous RNA but no 18 S or 28 S ribosomal RNAs and their buoyant density in CsCl was 1.42-1.45 g/cm3, suggesting that they were nuclear informosome-like particles released during incubation. 2. When the same nuclei were incubated in the same medium fortified with dialyzed cytosol, spermidine and yeast RNA (medium II), release of labeled 60-S and 40-S particles was observed. Using CsCl buoyant density gradient centrifugation, two components were found in the labeled ribonucleoprotein particles released from nuclei in this medium. The labeled 60-S particles were found to contain 28-S RNA as the main component and their buoyant density in CsCl was 1.61 g/cm3, suggesting that they were labeled large ribosomal subunits. The labeled 40-S particles contained both 18 S RNA and heterogeneous RNA and they formed two discrete bands in CsCl, at 1.40 and 1.56 g/cm3, suggesting that they contained small ribosomal subunits and nuclear informosome-like particles. 3. These results clearly indicate that addition of dialyzed cytosol, spermidine and low molecular yeast RNA to medium I causes the release of ribosomal subunits or their precursors from isolated nuclei in the in vitro system.