The metabolic fates of N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) and N,N-dibutylnitrosamine (DBN) were investigated in the rat and other animal species, to elucidate a possible relationship between metabolism and organotropic carcinogenicity to the urinary bladder of these N-nitrosamines. The principal urinary metabolite of BBN as well as of DBN in the rat was N-butyl-N-(3-carboxypropyl)nitrosamine (BCPN), which was demonstrated to be the active form of these compounds as bladder carcinogen. The species difference in response to BBN or DBN is discussed on the basis of the urinary excretion rate of BCPN. Metabolism in vivo and carcinogenicity of a number of BBN analogues were investigated in the rat and a general scheme for biotransformation of N-alkyl-N-(omega-hydroxyalkyl)nitrosamines is given. A possible correlation of structure and metabolism with organotropic carcinogenicity of BBN analogues is discussed, with special reference to selective induction of urinary bladder tumors.