A single oral 200 mg dose of tritium-labeled alpha,alpha,alpha-trifluoro-2-methyl-4'-nitro-m-propionotoluidide (flutamide) was given to 3 men. Analysis of plasma, urine and feces shows flutamide is rapidly and completely absorbed and excreted mainly through the kidneys. Analysis of plasma radioactivity shows flutamide is rapidly and extensively metabolized--only 2.5% of plasma radioactivity 1 h after dosing is associated with flutamide. At least 10 other metabolites are present, of which 6 have been tentatively identified. These are alpha,alpha,alpha-trifluoro-4'-amino-m-acetotoluidide (A); alpha,alpha,alpha-trifluoro-4'-amino-2-methyl-m-lactotoluidede (B); alpha,alpha,alpha-trifluoro-4'-nitro-m-acetotoluidede (C); alpha,alpha,alpha-trifluoro-2-methyl-4'-nitro-m-lactotoluidide (D); alpha,alpha,alpha-trifluoro-4'-amino-2-methyl-m-propionotoluidide (E); and alpha,alpha,alpha-trifluoro-6-nitro-m-toluidine (F). (D) represents 23% of plasma tritium 1 h after drug and is a major metabolite at all other measured times. Each of the other metabolites accounts for less than 10% of plasma radioactivity. Minor amounts of flutamide and (A) through (F) are found in 0-24-h urine. An additional metabolite, alpha,alpha,alpha-trifluoro-2-amino-5-nitro-p-cresol, constitutes 25% of urine tritium. The rapid conversion of flutamide to (D) and the high plasma levels of (D) suggest (D) might be the active form of flutamide.