The metabolism of pyrimidine nucleosides was found to be similar to the dissimilation of purine nucleosides. Thus pyrimidine base is cleaved from the nucleoside and is accumulated. Ribose disappears from the reaction mixture; it is rapidly oxidised to carbon dioxide and water;Pyrimidine-bound ribose cannot be determined by the orcinol test of Mejbaum (1939). A modified orcinol reaction for this purpose was developed and used in the study of pyrimidine riboside metabolism;Pyrimidine nucleoside phosphorylase was isolated from extracts of E. coli and brought to a high degree of purity by fractional precipitation with ammonium sulfate and adsorption on and elution from alumina C-gamma. The purified enzyme shows a six-hundred-fold increase in activity over that of the cell-free extract from which it was prepared;Pyrimidine nucleoside phosphorylase is a phosphorolytic enzyme. Inorganic phosphate is necessary for its action. Ribose-1-phosphate is accumulated as a result of its reaction upon nucleosides. The presence of ribose-1-phosphate was determined by analytical methods and by isolation as barium salt;The enzyme is highly specific. It does not attack cytidine. It is inactive against purine nucleosides. It will not split thymine desoxyriboside. Cytosine, thymine, and orotic acid do not react with ribose-1-phosphate in the presence of the enzyme. The role of orotic acid in the anabolism of pyrimidine nucleosides is discussed;Uridine was synthesised. Uracil when incubated with ribose-1-phosphate in the presence of pyrimidine nucleoside phosphorylase was converted to uridine. The equilibrium between uridine synthesis and splitting was determined.