To test the validity of the hypothesis that hypomethylation of DNA plays an important role in the initiation of carcinogenic process, 5-azacytidine (5-AzC) (10 mg/kg), an inhibitor of DNA methylation, was given to rats during the phase of repair synthesis induced by the three carcinogens, benzo[a]-pyrene (200 mg/kg), N-methyl-N-nitrosourea (60 mg/kg) and 1,2-dimethylhydrazine (1,2-DMH) (100 mg/kg). The initiated hepatocytes in the liver were assayed as the gamma-glutamyltransferase (gamma-GT) positive foci formed following a 2-week selection regimen consisting of dietary 0.02% 2-acetylaminofluorene coupled with a necrogenic dose of CCl4. The results obtained indicate that with all three carcinogens, administration of 5-AzC during repair synthesis increased the incidence of initiated hepatocytes, for example 10-20 foci/cm2 in 5-AzC and carcinogen-treated rats compared with 3-5 foci/cm2 in rats treated with carcinogen only. Administration of [3H]-5-azadeoxycytidine during the repair synthesis induced by 1,2-DMH further showed that 0.019 mol % of cytosine residues in DNA were substituted by the analogue, indicating that incorporation of 5-AzC occurs during repair synthesis. In the absence of the carcinogen, 5-AzC given after a two thirds partial hepatectomy, when its incorporation should be maximum, failed to induce any gamma-GT positive foci. The results suggest that hypomethylation of DNA per se may not be sufficient for initiation. Perhaps two events might be necessary for initiation, the first caused by the carcinogen and a second involving hypomethylation of DNA.