o-Toluidine hydrochloride and one of its metabolites, o-nitrosotoluene, were administered in the diet (0.028 mol/kg diet) to 2 groups of 30 male F-344 rats for 72 weeks. o-Nitrosotoluene induced significantly more tumors of the bladder ( 16 30 rats) and liver ( 20 30) than did o-toluidine hydrochloride (bladder, 4 30; liver, 3 30). Both compounds induced comparable numbers of peritoneal tumors and fibroma of the skin and spleen. o-Toluidine hydrochloride induced more mammary tumors ( 13 30) than did o-nitrosotoluene ( 3 30). The results indicate that N-oxidation is important in the induction of bladder and liver tumors by these single ring compounds but that other mechanisms could be involved in the induction of peritoneal, skin, spleen and mammary tumors.