Five beta-D-ribofuranosyl cyclic urea nucleosides (14-18), ranging in ring size from five to eight membered, were synthesized and evaluated as cytidine deaminase (CDA) inhibitors. The precursor protected nucleosides (9-13) were prepared by a condensation procedure utilizing persilylated ureas with a halo sugar under the specific catalytic activity of a HgO/HgBr2 mixture which provided exclusively the beta-anomers. Catalytic hydrogenation of known 1-(2,3,5-tri-O-benzoyl-beta-ribofuranosyl)-1,2-dihydropyrimidin-2-one (19) afforded nucleoside 10 identical with that obtained by the mercury-catalyzed condensation procedure. CDA activity varies significantly with the ring size of the urea aglycon the reaches its maximum level for the seven-membered analogues 16 and 17. The unexpected high potency of nucleoside 17 (Ki = 2.5 X 10(-8) M, human liver enzyme) is reported. This compound represents the most potent inhibitor of human liver CDA yet discovered.