Modifying effect of fermented brown rice by Aspergillus Oryzae (FBRA) during the initiation or post-initiation phase of N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumorigenesis was investigated in rats. Five-week-old male F344 rats were divided into 7 groups, and groups 1-5 were given subcutaneous injections of NMBA (0.5 mg/kg body weight/injection 15 times) for 5 weeks starting at 7 weeks of age. Groups 2 and 4 were fed the diet containing 5 and 10% FBRA during the initiation phase, respectively, whereas groups 3 and 5 were fed these diets during the post-initiation phase. Group 6 was given the diet containing 10% FBRA throughout the experiment, and group 7 was kept on the basal diet alone and served as an untreated control. Incidence and multiplicity of esophageal neoplasms of group 1 (NMBA alone) were 89% and 1.63+/-1.01/rat, respectively. Those of groups 3 (65%, 1.04+/-1.04) and 5 (58%, 0.77+/-0.86) were significantly less than those of group 1. Furthermore, the incidence and multiplicity of esophageal preneoplastic lesions (dysplasia) of group 5 were less than those of group 1. Post-initiation feeding of 10% FBRA significantly decreased BrdU incorporation in the non-lesional esophageal tissues when compared to that of the control. In addition, the analysis of expression levels of phase I enzymes of livers at the termination of experiment showed no clear differences among the groups. These observations indicate for the first time that FBRA inhibits NMBA-induced esophageal tumor development in rats possibly through inhibition of cell proliferation in the post-initiation phase, and suggest that FBRA is a promising dietary agent for prevention of human esophageal cancer.