1. Fragments of isolated rat liver plasma membrane possess a ribonuclease activity which at pH 7.8 in the presence of 10 mM EDTA can digest polyuridylic acid (poly(U)) and polycytidylic acid (poly(C)) but not polyadenylic acid (poly(A)) and polyguanylic acid (poly(G)). Under these conditions, the membrane preparation does not degrade native or denatured DNA. 2. The products of the reaction with poly(U) (10 mM EDTA present) can be separated on DEAE-Sephadex into oligonucleotides of increasing chain length. Most of the products are di- to hexa-nucleotides which contain terminal 3'-phosphate groups. 3. When EDTA is not present (pH 7.8 or 8.8) the plasma membrane preparation degrades both poly(A) and poly(U). With poly(A) the product is all nucleoside while with poly(U) as substrate most of the product is nucleoside, but also some oligonucleotides are produced. 4. The ribonuclease releases acid soluble products very slowly from high concentrations of poly(U) (mg/ml). 5. Uridine trinucleotide with and without a terminal 3'-phosphate group is degraded by rat liver plasma membrane. The trinucleotide diphosphate is rapidly hydrolyzed to nucleoside while the trinucleotide itself is slowly digested and yields intermediate products, including nucleoside.