The time courses of fecal excretion and tissue distribution of metabolites derived from 2,2',4,5,5'-pentachlorobiphenyl (CB101) and 2,2',3,4',5',6-hexachlorobiphenyl (CB149) were investigated in male Wistar rats. The metabolism of both congeners involved primarily hydroxylation at the 3-position, and methylthiolation at the 4-position. Metabolites distributed in tissue were dominated by different ratios of 3- and 4-methylsulfonyl (MeSO2) metabolites. The 3-/4-MeSO2 metabolite ratios in liver and adipose tissue for both congeners were 0.41-0.61 at day 4, and then increased to 0.85-1.00 for up to day 42. In contrast, the ratios in lung were 0.03-0.04, and then decreased to 0.01. Compared to the unchanged PCBs at day 42, the distribution ratios of 3-MeSO2 metabolites were greater in the order of liver (0.46 for CB101 and 0.21 for CB149) > kidney > blood > lung > adipose tissue, whereas those of 4-MeSO2 metabolites were in the order of lung (9.50 for CB101 and 4.00 for CB149) > kidney > blood > liver > adipose tissue, indicating the different binding affinity of 3-MeSO2 metabolites in liver from that of 4-MeSO2 metabolites in lungs of rats. Furthermore, the structure-tissue affinity relationship for 3-MeSO2 metabolites was investigated, following the administration of 11 3-MeSO2-PCB congeners to rats. The results indicated that the retention potential of 3-MeSO2 metabolites in the liver largely depends on the ortho-chlorine substitution in the biphenyl ring rather than the degree of chlorination.