1. The metabolism of the thiocarbamate herbicide SUTAN (butylate) was studied after administration of single oral doses of [isobutyl-1-14C]SUTAN to male and female rats. 2. The radiolabelled dose was rapidly absorbed and excreted, with 79% of the dose excreted in the urine in 72 h. The small percentages of radioactivity excreted in the faeces and as 14CO2 were significantly higher (P less than or equal to 0.05) in males than in females. 3. SUTAN was extensively metabolized, and no unmetabolized SUTAN was found in the urine. A total of 18 of the 29 urinary metabolites were identified, and identified metabolites represented 83-88% of the urinary radioactivity. 4. Diisobutylamine was the major urinary metabolite in both males and females, averaging 51% of the urinary radioactivity. 5. Other significant urinary metabolites included primary hydroxylated and tertiary hydroxylated diisobutylamines and a series of mercapturic acid pathway metabolites, including an S-glucuronide and several hydroxylated and unhydroxylated mercapturates. 6. Oxidations at the three alkyl groups produced a variety of minor urinary metabolites, and hydroxylation of the primary or tertiary carbon on the isobutyl groups, followed by an intramolecular reaction, generated a series of minor cyclized metabolites.