The combined effects of low doses of various carcinogens and carcinogenesis modifiers on tumor development were investigated by using a wide-spectrum organ carcinogenesis model in F344 rats. These agents were administered as three groups: (1) a group of known hepatocarcinogens; (2) a group of nitroso compounds having various target organ specificities; and (3) a group of antioxidants having various inhibiting or enhancing activities depending on the target organ. Doses were used which were generally below the known effective level for the individual chemical. These groups of chemicals were administered with or without prior administration of N-diethylnitrosamine (100 mg/kg body wt., i.p.), N-methylnitrosourea (4 x 20 mg/kg body wt., i.p.) and dihydroxy-di-N-propylnitrosamine (0.1% in drinking water for 2 weeks). The hepatocarcinogen group in combination with various nitroso compounds increased the incidences of liver hyperplastic nodules and hepatocellular carcinomas. In contrast, incidences were clearly reduced when the hepatocarcinogens and/or the nitroso compounds were administered in combination with the antioxidants. For the urinary bladder, the combination with nitroso compounds and antioxidants enhanced cancer development, and the addition of hepatocarcinogens further increased tumorigenesis. For the glandular stomach, additive effects on the numbers of pepsinogen isozyme 1-altered pyloric glands, a putative preneoplastic lesion, were produced by the combination treatment of antioxidants and the nitroso compounds. No synergistic effects on tumor development were seen in other organs. The results of the present study demonstrated that combinations of various compounds at low doses can additively or synergistically exert either enhancing or inhibitory effects on the development of preneoplastic and neoplastic lesions in different organs in a single model having a wide spectrum of organ effects.