Abstract 1. The metabolism of a wide range of di-, tri-, tetra-, penta- and hexachlorobiphenyls by hepatic microsomes isolated from control animals and animals treated with Aroclor 1254 was studied. 2. Hepatic microsomes isolated from control rats expressed higher rates of oxidations than avians. 3. Treatment of rats and pigeons with Aroclor 1254 induced cytochrome P450 dependent mono-oxygenases leading to an increased regioselective metabolism of PCB isomer and congeneres. 4. There was an inverse relationship between the degree of halosubstitution and microsomal oxidation. Meta-para carbon atoms free of halosubstitution were the preferred side for oxidation. 5. A good correlation was found between the in vitro metabolism of PCBs and their relative abundance in tissue extracts, thus suggesting oxidative metabolism to be the major route of metabolic disposal.