Tncubation of rat liver parenchymal cell suspensions at 37" results in degradation, to acid-soluble material, of 15% of cellular RNA at 30 minutes and 40% at three hours, beyond which there is little, if any, further degradation. The RNA which remains in the acid-insoluble form in the cells up to 30 minutes appears to exist largely in the native state. However, after 30 minutes, the acid-insoluble R N A of the cells is found to be partially dcpolymerised. These observations suggest the activation of a n intracellular nuclease on dispersion of the liver tissue to a single cell suspension and incubation at 37". This iiuclease appears to be responsible also for the dcgradation, reportcd earlier, of exogenous RNA taken up by the cells. Activation of the nucleasc is not due to depletion of pool of ATI' or of other ribonucleotides from the cells, during either dispersion of the tissue or incubation at 37". Incubation of the cells at 2 8 " , or of liver slices at 37"; does not lead to any significant degradation to acid-soluble material, or to partial depolymerisation, of RNA. Analysis of RNA obtained from cells incubated at 37" for various periods showed that chromatography on methylated albumin-kiesulguhr (MAK) and Sephadex columns is not suitable for detecting partial depolymerisation of cellular RNA; RNA shown to be partially depolymerised by analysis on sucrose density gradient, in an analytical ultracentrifuge, and on a cellulose column, gave the normal pattern in MAK or Sephadex i-uns.