The metabolism of 2-methyl-3-o-tolyl-4 (3H)-quinazolinone-N-oxide in rabbits was investigated in vivo and in vitro. In vivo metabolism, 2-methyl-3-o-tolyl-4 (3H)-quinazolinone, 2-methyl-3-o-hydroxy-methylphenyl-4 (3H)-quinazolinone and 2-nitrobenzo-o-toluidide were found in the urine receiving 20 mg/kg of 2-methyl-3-o-tolyl-4 (3H)-quinazolinone-N-oxide, which have been considered to be a possible intermediate of 2-nitrobenzo-o-toluidide. In vitro metabolism, 2-methyl-3-o-tolyl-4 (3H)-quinazolinone-N-oxide was reduced preferentially to 2-methyl-3-o-tolyl-4 (3H)-quinazolinone (about 50-60%), while 2-nitrobenzo-o-toluidide was not detected. When the incubation of 2-methyl-3-o-tolyl-4 (3H)-quinazolinone-N-oxide with the liver preparation was carried out in oxygen, the formation of 2-nitrobenzo-o-toluidide was confirmed by thin-layer chromatography.