14C-Nitralin (4-methylsulphonyl-2,6-dinitro-N,N-dipropyl-[14C]aniline) was dosed orally to rats (50 mg/kg). The radioactivity was quantitatively recovered (99.8%) between 0 and 72 h after dosing. Most of this was distributed equally between the urine and the faeces; about 1.5% was found in the carcass, skin and intestines. Structural analysis of the urinary metabolites showed that the metabolism proceeded via a complex multistep series of biotransformations involving nitro-group reduction, side-chain oxidation and removal and the subsequent formation of a variety of heterocyclic ring structures. A major metabolite was 7-amino-2-ethyl-5-methylsulphonyl-1-propylbenzimidazole; however, this accounted for only three-four per cent of the ingested radioactivity and was one of about five metabolites produced in this yield. Seven other metabolites were identified, none of which was derived from single-step biotransformation of nitralin. The latter, simpler, metabolites were detected on the incubation of nitralin with rat-liver enzymes of the type used in microbial mutagenicity testing. Two pathways (nitro-reduction and N-dealkylation) were observed under aerobic conditions but the former greatly predominated under anaerobic conditions.