1. Single doses of 3,3',4,4'-tetrachlorobiphenyl (TCB) were administered intraperitoneally to the male Wistar rat for metabolism studies. 2. Seven sulphur-containing metabolites of TCB were isolated from faeces, in addition to previously reported 4-hydroxy-3,3',4',5-tetrachlorobiphenyl and 5-hydroxy-TCB. Major sulphur-containing metabolites were 5- and 6-methylthio-TCBs, and 6-methylsulphonyl-3-methylthio-3',4,4'-trichlorobiphenyl. 3. The faecal excretions of hydroxy metabolites, methylthio metabolites and unchanged TCB accounted for 7.1, 0.5 and 0.3% of the dose respectively within 4 days after administration. 4. 5-Methylsulphonyl-TCB was detected and selectively retained in liver. The concentration ratio of 5-methylsulphonyl-TCB and unchanged TCB in liver was 1:4. 5. Following administration of 5- and 6-methylsulphonyl-TCBs to rat, 5-methylsulphonyl-TCB was localized in liver, whereas 6-methylsulphonyl-TCB was rapidly biotransformed to 6-methylsulphonyl-3-methylthio-3',4,4'-trichlorobiphenyl and excreted in the faeces.