Full clinical utility of the antileukemic drug, 5-azacytidine (l), is hampered by its facile hydrolysis in aqueous formulations. The present study sought to improve the stability of the parent drug while retaining the antitumor attributes through the synthesis of a reduced analogue of 1. Borohydride reduction of 1 gave 5,6-dihydrc-5-azacytidine hydrochloride (5) after acid hydrolysis of a boron-containing intermediate. The structure proof and characterization of 5 was achieved primarily with UV, NMR, and GC-MS with the aid of a deuterated derivative (7) prepared by using borodeuteride in the initial reduction step. Vigorous treatment of 5 with acid gave the aglycon 9 which was independently synthesized from 5-azacytosine (11). The dihydro analogue 5 was completely stable at room temperature in aqueous solutions over a broad pH range for up to 3 weeks. In comparative antitumor assays 5 showed good activity in L1210 systems when administered intraperitoneally or orally. Although higher dose levels were necessary, 5 had approximately 80% of the antitumor efficacy shown by 1. Neither 5 nor 1 showed a dependency on administration schedule. Cross resistance between 5 and 1 was demonstrated using an L1210 subline resistant to 1. 5 was found to be superior to 1 in therapeutic index and in its ability to cross the blood-brain barrier in sufficient quantity to be therapeutic against intracranially implanted L1210 cells. Subjective evidence is given which suggests 5 is a prodrug of 1.