Penetration of exogenous RNAases, native and albumin polymer-bound, from circulation into the brain parenchyma was studied. Native pancreatic RNAase was found to extend through the hematoencephalic barrier; maximal enzymatic activity was detected within 15-30 min after administration into a rat tail vein. The enzyme activity was not observed within 90 min both in blood serum and brain tissue. The conjugate of RNAase and human albumin with molecular mass of 300,000 penetrated through the hematoencephalic barrier. Maximal enzymatic activity was estimated within 3-5 hrs after administration and maintained rather high within 24 hrs in blood and brain parenchyma. This suggests that the preparations of this kind are promising in experimental medicine.