1. Bile secreted from rats given single oral doses of 2-chloro-N-isopropylacetanilide (propachlor) contained 58% dose as metabolites from the mercapturic acid pathway (glutathione, mercapturate, cysteine conjugates, and a sulphoxide of the mercapturate). 2. Bile secreted from rats given single oral doses of the cysteine conjugate of propachlor contained glucuronide conjugates of hydroxylated 2-methylsulphonyl-N-isopropylacetanilides. 3. In contrast, when the intestinal microflora were bypassed by intravenous administration of the cysteine conjugate of propachlor, the bile contained only the mercapturate and the sulphoxide of the mercapturate. 4. Rats fed an antibiotic-containing diet and given single oral doses of either propachlor or the cysteine conjugate of propachlor excreted only mercapturic acid pathway metabolites in the urine, bile, and faeces, and no methylsulphonyl-containing metabolites. Faecal 14C from the antibiotic-fed rats given either propachlor or the cysteine conjugate of propachlor was extractable, in contrast to previously reported unextractable faecal 14C residues from untreated rats given propachlor orally. 5. From these results, we conclude that metabolism by the microflora was necessary for production of the methylsulphonyl-containing metabolites excreted by the rat. Enterohepatic circulation of the xenobiotic moiety of these mercapturic acid pathway metabolites is influenced by the presence of a microbial C-S lyase.