Synopsis Four nitrofurans were compared for their effect on carcinogenesis induced by 4-(dimethylamino)azobenzene (DAB). The nitrofurans tested were 2-(2-furyl)-3- , 5-nitro-2-furaldehyde semicarbazone (N3), and 5-morpholinomethyl-3-(5-nitrofurfurylideneamino)-2-oxazolidone (N4). Male Donryu rats were maintained on a diet containing 0.06% DAB and 0.2% of a nitrofuran until they consumed 1g of DAB (about 3.5 months), and succeedingly on a basal diet for 1 month. The liver of these animals was

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... pared histopathologically with those of the animals fed a diet containing DAB alone. The induction of histopathological changes in the liver due to the administration of DAB was almost completely inhibited by the concurrent administration of N1 or N2. A similar activity was noted in N3, but the activity was moderate, and hyperplasia of small bile ducts and regenerative cell nodules were seen in the liver of the animals fed DAB and N3. No such inhibitory activity was found in N4, and typical trabecular hepatoma, cholangiofibrosis and atypical cell growths were seen in the animals fed DAB and N4. The liver enzyme activity of metabolizing azo dye was not significantly changed by 3.5-months' administration of any of the nitrofurans. The effect of nitrofurans on the levels of free and protein-bound dyes in the liver was more important for the mode of their action in hihibiting DAB carcinogenesis. However, it was noted that N2 and N3, both of which inhibited DAB carcinogenesis, showed no significant activity to reduce the protein-bound dye level.