There is much evidence in the literature to indicate extrahepatic conjugation of naphthol by various isolated tissue preparations. Using in vivo methodology, where multiple sites of input are employed, it has been demonstrated that both lung and intestinal mucosa make a significant contribution in the conjugation of naphthol in the rat. Under the conditions studied, both of the above organs (pulmonary extraction ratio 0.44, intestinal extraction ratio 0.46) are more than twice as effective as the liver in eliminating naphthol (hepatic extraction ratio 0.18). This multiple sites of input method has been compared with a multiple sites of sampling method. Similar results have been obtained in quantifying pulmonary conjugation of naphthol. The relevant merits of the two approaches are discussed.