1. The biotransformation of a single i.p. dose of [14C]2-chloroaniline (1.0 mmol/kg, approximately 60 microCi/rat) was investigated in the urine and faeces of the male Fischer 344 rat. 2. During 24 h, 53.1% of the administered radioactivity was eliminated into the urine, while < 1% of the radioactivity appeared in the faeces. 3. The major biotransformation pathways were para-hydroxylation and sulphate conjugation. 4-Amino-3-chlorophenyl sulphate was the major urinary metabolite comprising 31.6% of total urinary radioactivity. The para-hydroxylated metabolite, 4-amino-3-chlorophenol (10.8%), and its O-glucuronide conjugate (3.7%) were also urinary metabolites. The formation of direct conjugates of 2-chloroaniline, the N-sulphate and N-glucuronide, was significant with as much as 18.6 and 8.6%, respectively, of these metabolites excreted in the urine. The parent compound, 2-chloroaniline, accounted for 16.9% of urinary radioactivity. 4. N-Acetylated products were minor metabolites present in urine as 2-chloro-4-hydroxyacetanilide and its sulphate or glucuronide conjugate. Neither 2-chloroacetanilide nor its oxidation products, 2-chloroglycolanilide and 2-chlorooxanilic acid, were urinary metabolites.