Conventional rats metabolize 2-chloro-N-isopropylacetanilide (propachlor) mainly to various 2-methylsulphonylacetanilides and to residues in the faeces that are unextractable. Mercapturic acid pathway (MAP) metabolites of propachlor are the source of the methylsulphonyl sulphur and the insoluble faecal residues. Germ-free and antibiotic-treated rats quantitatively metabolized propachlor only to MAP metabolites, which were excreted in the urine and faeces (all water-soluble). Germ-free rats given enemas of caecal contents from conventional rats were qualitatively similar to conventional rats with respect to the metabolism of propachlor within 56 days of the inoculations i.e. they excreted 2-methylsulphonyl acetanilides in the urine and insoluble residues in the faeces. They approached quantitative similarity within 180 days. Antibiotic-treated rats spontaneously recovered the ability to metabolize propachlor as conventional rats, qualitatively within 6 days and quantitatively within 21 days. While differences occurred in the faecal metabolites of propachlor, biliary secretion of metabolites by germ-free rats was not different from that of conventional or antibiotic-treated rats.