In mammals, PCBs are readily absorbed through the gut, respiratory system, and skin. Initally, PCBs concentrate in liver, blood, and muscle; eventually, accumulations are highest in adipose tissue and skin. Phenolic derivatives or dihydrodiols are the major metabolites, but susceptibility of individual PCB isomers to metabolism is a function of the number of chlorine atoms present on the biphenyl rings and their arrangement. In general, most readily metabolized PCBs are also rapidly excreted in urine and bile. The highly chlorinated isomers are difficult to metabolize and accumulate almost indefinitely. PCBs can be transferred to young mammals either transplacentally or in breast milk. Retention of PCBs is highly species specific: nonhuman primates, for example, retained PCBs more efficiently than rodents (EPA 1980). PCB patterns, especially in warm-blooded animals, only vaguely resemble the mixture from which they originated (Hansen et al. 1983; Ernst 1984). Subtle differences between chlorobiphenyls are further amplified by differences among various animals to absorb, distribute, biotransform, and excrete individual PCB isomers (Hansen et al. 1983). 

Biological activities of PCB isomers differ substantially. Among three symmetrical hexachlorobiphenyl (HCBP) isomers, 3,4,5,3',4',5'-HCBP was the most toxic in dietary lethal studies to domestic chickens (Gallus spp.), 2,4,5,21,4',5'-HCBP was least toxic, and 2,4,6,2',4',6'-HCBP intermediate in toxicity (Ayer 1976). A tetrachlorobiphenyl (3,4,3',4') was more than 1000 times more potent in producing effects in chicks than 2,4,5,2',4',5'-HCBP both as an hepatotoxin and as an inducer of cytochrome P mediated mixed function oxidases (Rifkind et al. 1984). The biological half-life of intravenously administered PCB formulations in liver of rat (Rattus spp.) increased with increasing chlorination: half-lives of 86 hours were determined for 4-monochlorobiphenyl, 99 hours for 4,4'-dichlorobiphenyl, 193 hours for 2,2',4,5,5'-pentachlorobiphenyl, and 1,308 hours for 2,2',4,4',5,5'-hexachlorobiphenyl (Menzie 1978). 

PCBs are usually taken up by animals and stored in lipids under circumstances of increasing lipid content in organs. However, recent studies with marine fishes indicate that PCB components remain mobilizable from organs whose lipid contents increased (Boon et al. 1984). The degree of mobilization in codfish (Gadus morhua) and sole (Solea solea) appeared to be related to polar lipid components such as phospholipids and glycerols. In other aquatic species, the role of diet and tissue specific sites are important. The patterns of pentachloro BP and higher chlorinated BP (not present in seawater) were equal in marine clams, worms, and sediments, and strongly indicate that uptake was via the diet or from sediments (Duinker et al. 1983). In freshwater fishes, direct partitioning across the gill membrane of the blood:water interface controls PCB accumulation; however, dietary PCBs may significantly affect accumulation and exchange rates at the gill membrane (Rohrer et al. 1982).