The brain ribonucleases of rabbit, guinea pig, rat, mouse and gerbil were investigated by histochemical and biochemical methods. For the localization, the ribonucleases were electrophoretically transferred from cryostat sections to polyacrylamide gels. Elevated ribonuclease activities were found in the cortex, the basal ganglia, the hippocampal formation and the ventricles, whereas the corpus callosum and the internal capsule exhibited lower activities. The total RNA degrading activities of the brain extracts of the different species varied in a wide range. However, a pre-requisite for the measurement of acid soluble degradation products in the test system was the inactivation of endogeneous ribonuclease inhibitors, present in all extracts. Molecular weight analysis by means of SDS-polyacrylamide gel electrophoresis revealed a characteristic set of ribonucleases for each species, consisting of enzymes with different pH-optima.