The effects of antioxidant administration during the post initiation phase of gastric tumor development were investigated in male Wistar rats treated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Animals (20/group) were given MNNG in the drinking water (100 mg/l) for 8 weeks, and for the duration of this treatment were also fed on diet supplemented with 10% sodium chloride. Thereafter, they were divided into 6 groups and were maintained on diet containing either 2% butylated hydroxyanisole (BHA), 1% BHA, 1% butylated hydroxytoluene (BHT), 1% ethoxyquin (EQ) or 1% DL-alpha-tocopherol (alpha-TP) for 32 weeks. A carcinogen control group was fed the basal diet without antioxidant supplementation. The experiment was terminated 40 weeks after the beginning of administration of MNNG and development of gastroduodenal tumors was determined histopathologically. EQ significantly increased the incidence of tumors in the glandular stomach. No modification of tumor development in this region of the organ were observed with 2% BHA, 1% BHA, 1% BHT or 1% alpha-TP, although both 2% BHA and 1% BHA induced and/or promoted tumor development in the forestomach. In addition, nephrocalcinosis was identified only in the kidneys of rats given EQ after MNNG treatment.