Female Wistar rats were given a single oral dose of aflatoxin B1, either alone or with a large amount of riboflavin. Biochemical and histologic studies were performed for 30 months. Nine animals of 19 in the aflatoxin-treated group and only 5 of 18 in the riboflavin-aflatoxin-treated group developed hepatomas. The number of rats was insufficient for tests of statistical significance to be fruitful. Urinary excretion of tryptophan metabolites was studied in aflatoxin- and riboflavin-treated rats after an oral administration of 10 mg tryptophan/100 g rat. Riboflavin did not affect the percentage of aflatoxin-treated animals with abnormal urinary excretion patterns, but did increase the magnitude of the disturbances in elimination of kynurenic and xanthurenic acids. The hepatic tryptophan-oxygenase activity was increased only in the two groups given riboflavin, and the levels of nucleic acids were the same in all groups.