Workers exposed to high levels of benzidine have a 100-fold increased incidence of bladder cancer. This review evaluates the overall metabolism of benzidine to determine pathways important to initiation of bladder cancer. Upon incubation of benzidine with liver slices from rats, dogs, and humans, different proportions of this diamine were N-acetylated and N-glucuronidated. With dogs, a non-acetylator species, N-glucuronidation was the major pathway. In contrast, little glucuronidation was observed in rats with N, N'-diacetylbenzidine, the major metabolite of benzidine. Human liver slices demonstrated both extensive N-acetylation and N-glucuronidation. Differences between rats and humans were attributed to rapid deacetylation by human liver with N-acetylbenzidine rather than an accumulation of N, N'-diacetylbenzidine. N-Acetylbenzidine oxidative metabolism was also observed. The acid lability of glucuronide products of benzidine, N-acetylbenzidine, and oxidation products of N-acetylbenzidine metabolism was assessed. N-Glucuronides of benzidine, N-acetylbenzidine, and N'-hydroxy-N-acetylbenzidine were acid-labile, with the latter having a much longer half-time than the former two glucuronides. Because bladder epithelium contains relatively high levels of prostaglandin H synthase and not cytochrome P450, the peroxidative metabolism of N-acetylbenzidine was assessed. N'-(3'-Monophospho-deoxyguanosin-8-yl)-N-acetylbenzidine was the only DNA adduct detected. This adduct is also the major adduct detected in bladder cells from workers exposed to benzidine. In urine from these workers, an inverse relationship between urine pH and levels of free (unconjugated) benzidine and N-acetylbenzidine was observed. A similar inverse relationship was observed for urine pH and levels of bladder cell N'-(3'-monophospho-deoxyguanosin-8-yl)-N-acetylbenzidine. These results suggest multiple pathways (acetylation, glucuronidation, peroxidation) in multiple organs (liver, blood, kidney, bladder) are important in benzidine-induced bladder cancer.