The metabolic disposition of 14C-labeled nitromethaqualone was investigated in rats. Unlabeled nitromethaqualone was used for studies on humans. nitromethaqualone was eliminated from the body after most of it had undergone biotransformation. Both humans and rats reduced the nitro group of nitromethaqualone to the corresponding amino derivative, which was partially transformed to the corresponding acetylated form. Cleavage of the quinazolinone nucleus resulting in 2-methoxyl-4-nitroaniline was also observed in humans. In rats additional major metabolites arose from the oxidation of the 2-methyl group into hydroxymethyl resulting in 2-hydroxymethyl-3-(2'-methoxy-4-nitrophenyl)-4(3H)-quinazolinone and concomitant in vivo reduction of the latter resulting in 2-hydroxymethyl-3-(2'-methoxy-4'-aminophenyl)-4(3H)-quinazolinone. Both metabolites were also excreted as glucuronides. In rats fecal excretion accounted for 55-60% of the administered dose, while 24-27% was excreted in the urine. Protracted excretion in both humans and rats indicated an extensive enterohepatic circulation.