The effects of 4,4' -diaminodiphenylmethane (DDPM) administration in the "post-initiation" stage of liver, kidney and bladder carcinogenesis were examined in male F344 rats. In experiment 1, rats were given drinking water containing 0.1% N-ethyl-N-hydroxyethyl-nitrosamine for 2 weeks than diet containing 0.1% DDPM for 32 weeks. In week 3, the right kidney was removed. The incidence of hepatocellular carcinoma was significantly less in rats given DDPM than in controls. DDPM decreased the incidence and average number/cm2 of neoplastic nodules and renal cell tumors of the kidney. In experiment II, rats were given 0.01% N-butyl-N-(4-hydroxybutyl)nitrosamine for 4 weeks and then 0.1% DDPM for 34 weeks kin their drinking water. DDPM inhibited the induction of papillomas in the bladder. These results indicate that DDPM administration in the "post-initiation" stage inhibited liver, kidney and bladder carcinogenesis in rats.