Gene silencing by CpG island promoter hypermethylation has awakened the interest for DNA demethylating agents as chemotherapy drugs. Zebularine (1-[beta-D-ribofuranosil]-1,2-dihydropyrimidin-2-1) has been recently described as a new DNA methylation inhibitor. Here we have studied its effects in a mouse model of radiation-induced lymphomagenesis using nuclear magnetic resonance (NMR) and positron emission tomography (PET). All control animals presented large thymic T lymphomas and died between 4 and 5.5 months. In contrast, 40% (12 of 30) of zebularine-treated animals were still alive after 1 year (Kaplan-Meier P < .001). NMR and PET imaging showed that surviving animals presented a thymus structure/volume similar to normal mice of the same age. Most important, zebularine demonstrated a complete lack of toxicity in nonirradiated control mice. DNA hypomethylation induced by zebularine occurred in association with depletion in extractable DNA methyltransferase 1 protein. Thus, our data support the role of zebularine as a DNA demethylating agent with antitumor activity and little toxicity.