Repeated installation of N-hydroxy-2 naphthylamine into dog bladders produced tumors of this organ. There is a correlation of degree of carcinogenicity of 1-naphthylamine, 2-naphthylamine, and 4-aminobiphenyl with both methemoglobin production in the blood and concentration of N-oxidation products in the urine. Thus, N-hydroxylation appears to be the key to bladder cancer production by aromatic amines.