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