The metabolism of HCB in the isolated perfused rat liver was studied by administration of [14C]HCB diluted with unlabelled HCB at a total dose of 0.1 mg HCB/ml perfusate. Metabolites in bile, perfusate and liver were studied by GLC-mass spectrometry. Histological examination of the livers showed that no hepatic necrosis had developed, although there was a slight increase in ASAT and ALAT in the perfusate and about 50% decrease in hepatic glutathione. About 0.15% of administered radioactivity was recovered in the bile within 2 hr. In the bile, HCB together with the metabolites pentachlorothiophenol and pentachlorophenol, were identified and accounted for about 20% of the radioactivity excreted. In addition, eleven metabolites with 4 or 5 chlorines were isolated. In the perfusate and in the liver, unchanged HCB was responsible for most of the radioactivity. Traces of pentachlorothiophenol and pentachlorophenol were identified in the perfusate and the liver, respectively.