Abstract A reduced pyrimidine nucleoside, 1-(β- d-ribofuranosyl)-4-hydroxytetra-hydro-2(1H)-pyrimidinone, termed tetrahydrouridine, was found to be a potent and specific inhibitor of the enzymatic deamination of 1-β- d-arabinofuranosylcytosine (aracytidine, ara-C) by preparations of human liver. Kinetic studies were consistent with a model of partial (or regulatory-type) inhibition in which the inhibited enzyme was still able to form product, but at a reduced rate. Enzyme inhibition was dependent upon a preincubation reaction with tetrahydrouridine, and the substrate appeared to interact preferentially with uninhibited enzyme. The uninhibited and fully inhibited deamination reactions had measured K m constants of 1.6 × 10 −4 M and > 1 × 10 −2 M respectively; the ratio of their V max values was > 4. The dissociation constant ( K i ) of the enzyme-inhibitor complex was approximately 10 −4–10 −5 M. Tetrahydrouridine also markedly inhibited deaminase preparations from mouse kidney, rhesus monkey liver and actinomycete mycelium. Several other pyrimidine nucleosides, catalytically reduced by the same process that yields tetrahydrouridine, were also potent inhibitors of the deaminase. In preliminary studies in vivo in two dogs and two rhesus monkeys, tetrahydrouridine markedly inhibited the deamination of ara-C at nontoxic doses.