The formation of glutathione (GSH) conjugate in the detoxification of (1-14C)-naphthalene and (naphthyl-14C)-carbaryl was investigated using rat liver homogenate. The mercapturic acid conjugate in rats was also investigated by collection of urine after intraperitoneal injection of 14C substrates. The formation of water-soluble metabolites in vitro from naphthalene was dependent on the amount of glutathione added, but this was not seen in carbaryl metabolism. In vitro, the metabolism of (1-14C)-naphthalene produced 50% GSH conjugate in the incubation mixture, whereas in vivo the metabolism of this compound produced 65% mercapturic acid conjugate in the urine. There was no evidence of GSH or mercapturic acid conjugate in the metabolism of (naphthyl-14C)-carbaryl in vitro and in vivo. This conclusion was made by comparing the nature and chemical characteristics of GSH and mercapturic acid conjugates formed in (1-14C)-naphthalene metabolism. With the aid of the specific enzyme (e.g. B-glucuronidase and sulfatase) and acid hydrolysis, the water-soluble metabolites of (naphthyl-14C)-carbaryl were tentatively recognized as glucuronide or sulfate conjugated mainly with 5,6-dihydro-5,6-dihydroxycarbaryl or N-hydroxymethyl carbaryl and their hydrolytic products. This data demonstrated that the substituent group on the naphthalene molecule may affect the significance of GSH conjugation.